pattern to predict -ent vs -ant? [duplicate]

Yes, this is for real. No, there really is no rule. There used to be a rule in Latin, though. Etymonline explains in more detail:

-ance
suffix attached to verbs to form abstract nouns of process or fact (convergence from converge), or of state or quality (absence from absent); ultimately from L. -antia and -entia, which depended on the vowel in the stem word. As Old French evolved from Latin, these were leveled to -ance, but later French borrowings from Latin (some of them subsequently passed to English) used the appropriate Latin form of the ending, as did words borrowed by English directly from Latin (diligence, absence). English thus inherited a confused mass of words from French and further confused it since c.1500 by restoring -ence selectively in some forms of these words to conform with Latin. Thus dependant, but independence, etc.

There are four conjugations of verbs in Latin, verbs that end in -are, -ēre, -ere, and -ire. The four conjugations are still evident in Spanish, French, Italian, etc. In English nouns that derive from these verbs end with -antia, -entia, and -ientia, from which we eventually get our nouns ending with -ance, -ence, and -ience.

e.g. parlance is from parlare, insurgence is from insurgere, audience is from audire. So remembering the spelling in English helps if you studied Latin ;)

However, standard spelling in English wasn't a huge priority until people like Samuel Johnson started writing dictionaries, so there are all sorts of exceptions to these rules.

The following is meant to supplement not supplant existing answers. In general, you look at what it was in Latin; however, there are several prominent exceptions.

The Etymonline entry regarding this is cribbed and abbreviated from the OED. Here’s what the OED says about these.

‑ance and ‑ence

‑ance

a. Fr. ‑ance :– L. ‑ānt‑ia, ‑ēnt‑ia, ‑ent‑ia (see ‑ence), all of which in words that survived into Fr., or were formed in Fr. as nouns of action, on the pres. pple., were levelled under ‑ance. But other L. words of this form, subseq. adopted in Fr., took ‑ence or ‑ance, according to L. spelling. Thus of popular preservation or formation, aidance, assistance, complaisance, nuisance, parlance, séance; of later learned adoption from L., absence, clémence, différence, diligence, providence, prudence, as well as élégance, tempérance. Words of both classes were adopted in Eng. in their actual Fr. forms, which they still generally retain. But, since 1500, various words orig. in ‑ance from Fr. have been altered back to ‑ence, after L.; and all words recently adopted from L., directly or through mod.Fr., or formed on L. analogies, have taken ‑ence or ‑ance according to the L. vowel. Hence, mod.E. words in ‑ance partly represent L. ‑āntia, but largely L. ‑entia, ‑ēntia, through OFr. ‑ance; partly also mod.Fr. ‑ance from vbs. of various origin. On the other hand, OFr. ‑ance :– L. ‑entia, ‑ēntia, is, in consequence of refashioning, partly represented by Eng. ‑ence. For the confusion and inconsistency which this causes in current spelling, as in dependance, ‑dence, resistance, subsistence, see ‑ence. As, in many cases, the OFr. vbs. themselves, as well as their derivatives in ‑ance, were adopted in Eng. (e.g. appear ‑ance, assist ‑ance, purvey ‑ance, suffer ‑ance), the suffix became to a certain extent a living formative, and was occas. used to form similar nouns of action on native vbs., as abid‑ance, abear‑ance, forbear‑ance, further‑ance, hinder‑ance, ridd‑ance, etc. For meaning, see ‑ence; and cf. ‑ancy.

‑ence

a. Fr. ‑ence, ad. L. ‑entia, forming abstr. sbs., usually of quality, rarely of action, on ppl. stems in ‑ent‑, e.g. sapient‑em knowing, sapient‑ia knowingness, sapience; audient‑em hearing, audient‑ia the process of hearing, audience. As the ppl. stem had ‑ent‑, ‑ant‑, the derivative sbs. had ‑entia (prūdentia), ‑antia (īnfantia); but all these were levelled in OFr. to ‑ance, in words that survived in popular use, or were formed analogically on the pr. pple. in ‑ant; as aidance, assistance, complaisance, contenance, nuisance, parlance, séance. These were sbs. of action or process, the value with which the suffix was retained in Fr. as a living formative. But subsequently other L. words in ‑ntia, which had not survived in the living language, were readopted on the analogy of these, but with ‑ence or ‑ance according to the L. vowel, e.g. absence, clémence, diligence, élégance, présence, providence, prudence, tempérance, violence. These were sbs. of quality or state; all Fr. words in ‑ence are of this class. Both classes were adodpted in ME. in their actual Fr. forms and senses, which they generally still retain; but since 1500, some of those in ‑ance have been altered back to ‑ence after L. All words since adopted from or formed on L., follow L. precedent as to ‑ence or ‑ance. The result is that the modern spelling of individual words, and still more of groups of cogn. words, is uncertain and discordant; cf. assistance, consistence, existence, resistance, subsistence; attendance, superintendence; ascendant, ‑ent, ‑ancy, ‑ency, condescendence; dependant, ‑ent, ‑ance, ‑ence, independence; appearance, apparent; pertinence, appurtenance. In sense, words in ‑nce are partly nouns of action, as in OFr., partly of state or quality, as in L. The latter idea is more distinctly expressed by the variant ‑ncy (see ‑y = ‑ie :– ‑ia) which has been formed in Eng. as a direct adaptation of L. ‑ntia; see ‑ency, ‑ancy.

‑ant and ‑ent

‑ant

a. Fr. ‑ant, sometimes :– L. ‑entem, ‑āntem, ‑ēntem, ending of pres. pple. (see ‑ent); sometimes a later adaptation of ‑āntem only. All the participial forms were in OFr. levelled under ‑ant, the sole ending of the pr. pple., as L. amānt‑, vidēnt‑, sedēnt‑, crēdent‑em in Fr. amant, voyant, séant, croyant. But other words were subsequently adopted in their L. stem form, as prudent, présent, élégant. Hence Fr. words in ‑ant are of two kinds, one answering to L. ‑ānt, the other to L. ‑ent, ‑ēnt. All were adopted, in their actual Fr. forms, in Eng., where they subseq. became ‑ˈau.nt; then again, with the change of stress, ‑ant, as L. affīdent‑em, diffī‑dent‑em, plicānt‑em, servient‑em, tenēnt‑em, OFr. afiant, defiant, pliant, serjeant, tenant, ME. afiˈa(u).nt, defiˈa(u).nt, pliˈa(u).nt, serjeˈau.nt, teˈnau.nt. Most of them retain ‑ant, e.g. claimant, pleasant, poursuivant, servant, suppliant, valiant; but since 1500 some have been refashioned with ‑ent after L., wholly (as apparaunt, ‑ent), or partly (as in pendant, ‑ent, dependant, ‑ent, ascendant, ‑ent). Hence, inconsistency and uncertainty in the present spelling of many words, in which L. and Fr. analogies are at variance: see ‑ent. Many new words of this class have been adopted from L. ‑āntem directly or through later Fr., or have been formed on L. analogies, or adopted from mod.Fr. and Romance ‑ant, ‑ante; as concomitant, protestant, commandant, anæsthesiant. For sense, see ‑ent.

‑ent

a. Fr. ‑ent, ad. L. ‑ent‑em, the ending of pr. pples. of vbs. of the 2nd, 3rd, and 4th conjugation, as rīdent‑em, currentem, audientem. (In the pples. of the 3rd and 4th conjugation this ending represents OAryan ‑nt‑, or perh. ‑ent‑, of the ablaut‑series ‑ent‑, ‑ont‑, ‑nt‑; cf. Skr. ‑ant‑, ‑at‑, Gr. ‑οντ‑, Goth. ‑and‑, OE. ‑end‑; in those of the 2nd conjugation it represents this suffix combined with the thematic ‑e‑ of the vb.; similarly the ‑ant‑ of the 1st conjugation includes a thematic ‑a‑.) In OFr. this suffix and the corresponding ‑ant‑em of the 1st conjugation were levelled under ‑ant, the sole ending of the Fr. pr. pple., as riant, courant, mourant, levant ( :– L. levantem). At a later time many L. forms in ‑ent‑, which had acquired an adj. sense, were adopted in Fr. as adjs. with the ‑ent‑ unchanged, as diligent, évident; some of these were duplicates of living ppl. forms in ‑ant, as convénient = convenant, provident = pourvoyant, confident = confiant. The Fr. words in ‑ant, ‑ent, which were adopted into Eng., have generally retained the form of the suffix which they had in Fr.; but since 1500 there has been a tendency to refashion them after Lat., and hence several words in ‑ant have changed that ending for ‑ent, either entirely or in certain senses. In mod.Eng. also many Lat. words in ‑entem have been directly adopted, always in the form ‑ent. The conflict between Eng. and Fr. analogies occasions frequent inconsistency and uncertainty in the present spelling of words with this suffix; cf. e.g. assistant, persistent; attendant, superintendent; dependant, ‑ent, independent.

2. In sense the words in ‑ent, ‑ant are primarily adjs., sometimes distinctly ppl., as convergent, obsolescent, errant, peccant; some, however, are, like many words of the same type in Lat. and Fr., used as sbs. (either in addition to the adj. use or exclusively), meaning (a) a personal agent, as agent, claimant, president, regent; (b) a material agent, as coefficient, current, ingredient, secant, tangent, torrent; esp. in Medicine, as aperient, astringent, emollient, expectorant.

Well, there seems to have some rules to that in English according to a blogger I've found on Oxford Dictionaries:

-ance or -ence?

These endings are used to make nouns, meaning either: a quality or state (e.g. ignorance is the state of being ignorant about something) or an action (e.g. emergence is the action of emerging from somewhere).

One key way of knowing which ending to choose is to be aware of the words that these nouns are formed from. Some are made from verbs (e.g. performance from perform) and some are from adjectives (for example intelligence from intelligent).

-ance

If the word is formed from a verb that ends in -y, -ure, or -ear, then the ending of the noun will be spelled -ance, for example:

Verb: apply comply / Noun: appliance compliance

(...)

If the noun is related to a verb which ends in -ate, then the ending is likely to be -ance, for instance:

Verb: deviate dominate / Noun: deviance dominance (...)

A further tip is that, if the stem of the word (the part before the ending) ends in a ‘hard’ c (pronounced like the c in cab) or a ‘hard’ g (pronounced like the g in get), then the ending will be spelled -ance. For example: significance; elegance.

As for the other nouns which end in -ance, there are no hard-and-fast rules, so you may find it helpful to memorize them. Here’s a handy list of the most common:

ABUNDANCE acceptance acquaintance

(...)

-ence

If the word is formed from a verb ending in -ere, then the ending will be spelled -ence. For example:

Verb: adhere cohere / Noun: adherence coherence

(...)

So, you may ask, why is perseverance (from persevere) spelled -ance? Sorry, it’s simply an exception to this rule…

Does the related verb end in a stressed -er syllable? If yes, then -ence is the correct ending, as in:

Verb: confer infer / Noun: conference inference

(...)

Note that, although the verb differ is stressed on the first syllable rather than the final –er, difference is still spelled with -ence at the end. If the word contains the syllables -cid-, -fid-, -sid-, or -vid- immediately before the ending, the correct suffix is -ence. For instance:

CONFIDENCE evidence

(...)

If the stem of the word ends in a soft c (pronounced like the c in cell) or a soft g (pronounced like the g in gin), then the ending will be -ence. For example:

ADOLESCENCE indulgence

(...)

Again, some words don’t follow these rules (or the ‘rules’ have several exceptions, so can be rather difficult to learn). Here’s a list of some common nouns ending in -ence:

ABSENCE DEPENDENCE OBEDIENCE

(...)

And much more at: http://blog.oxforddictionaries.com/2013/05/ance-ence-suffixes/