-t- and -tt- in present participle and past participle of words

Solution 1:

When the 't' is a part of stressed syllable and is preceded by a vowel, it is doubled in present participle and past participle.

For example: Submit has two syllables, the 't' is a part of the second syllable which is stressed so it's doubled in its present participle and past participle.

Sub'mit -> submitting, submitted.

Similarly, admit has two syllables in which the second one is stressed and the 't' is a part of second syllable so it's doubled in its present participle and past participle.

Ad'mit -> admitting, admitted.

When the 't' is not a part of stressed syllable, it is not doubled in present participle and past participle.

For example: Elicit has three syllables. The 't' is a part of unstressed syllable so it's not doubled.

E'licit -> eliciting, elicited.

Similarly, interpret has three syllables. Stress is on the second syllable and the 't' is not a part of the stressed syllable so it's not doubled in present participle and past participle.

In'terpret -> interpreting, interpreted.

Verbs that have only one syllable and end with 't' can have -tt- or -t-, it depends on the sound that precedes the 't'.

• If a vowel precedes the 't' in one-syllable word, it has -tt- in its present participle and past participle.

For example: Spot -> spotted, fit -> fitted, jot -> jotted etc.

• If a consonant precedes the 't' in one-syllable word, it has -t- in its present participle and past participle.

For example: Act -> acted, want -> wanted, jolt -> jolted etc.

Multisyllabic verbs in which 't' is preceded by a consonant sound have -t- in their present participle and past participle.

Examples: Accept -> accepted, attempt -> attempted. interact -> interacted etc.

Solution 2:

I This is a regular feature of English spelling relative to one syllable words with an ending of the form "C-V-C", where C stands for a single graphic consonant and V a single graphic vowel (ref. 1, ref. 2). For these words and for the exceptional words with their ending of the form "QU-I-C" where C is a consonant (quit, quid, quip) the doubling of the last consonant occurs when constructing other words by addition of endings (inflections); these endings are first the following important ones.

-ER (comparative)
-EST (superlative)
-ING (progressive form)
-ED (past form)

• /æ/ ban, bannning, banned - man, manning, manned - pan, panning, panned - tap, tapping, tapped -
• /e/ wet, wetter, wettest - bet, betting - set, setting -

• /ɪ/ pit, pitted, pitting - fit, fitting, fitted - flit, flitting, flitted - thin, thinner, thinnest - whip, whipping, whipped - tip, tipping, tipped

• /ʌ/ cut, cutting, hum, humming, run, running - stun, stunning, stunned -

• /ʊ/ put, putting

• /ɒ/ pot, potted, potting - dot, dotted, dotting - rot, rotted, rotting - stop, stopping, stopped

• /ɑ:/ star, starring, starred -

• /ɔ:/ war, warring, warred
• /ɜ:/ blur, blurring, blurred stir, stirring, stirred - whir, whirring, whirred

Exceptions (some)

• ending in x: fix, fixing, fixed, fixer - sex, sexing, sexed -vex, vexing, vexed -

There are also other endings, some of them important, for which this rule is true. However, references can't be obtained for the time being and the results depend on my sole memory; moreover I suspect that there are more such endings.

-ER (noun formation: verb → noun)
-ING, INGS (noun formation: verb → noun)
-Y (adjective formation: noun → adjective)
-ED (adjective formation: noun → adjective)
-IE, Y (diminutives)

• rob, robber - rub, rubber - stop, stopper - rap, rapper - let, letter - rip, ripper - pot, potter …
• wed, wedding - fit, fittings …
• cat, catty - fat, fatty - jam, jammy - star, starry - fur, furry (notice how the doubling draws a clear difference from "fury") …
• fun, funny - run, runner …
• fur, furred …
• lad, laddie - Ben, Benny - Jim, Jimmy - Bob, Bobby - Rob, Robbie - Sam, Sammy - Tom, Tommy …

II The same rule is true for two syllable words when they are stressed on the last syllable (same refence as above). I should add that, as shown me through my own constatation, the conditions for the doubling of the consonant might not be met and the doubling still occur (panel, channel (AmE -l-), cancel (AmE -l-), …)

• /e/ dispel, dispelling, dispelled - repel, repelling - impel - compel - propel - rebel - revel

• /i/ begin, beginning, beginned, beginner - distil - fulfil, fulfilling, ("fulfilment" in BrE but "fulfillment" in AmE) - instil - admit - remit - submit - commit - equip, equipping, equipped -

• /ɜ:/ refer, referring, reffred - confer - infer - defer … Notice that the r is not doubled in "reference, conference, referable, inference, etc.).

• demur, demurring, demurred - incur, incurring, incurred

• /ɔ:/ abhor, abhorring, abhorred (notice "abhorrence" and "abhorrent" with two r's also)

Exceptions (some)

• ending in x: annex, annexing, annexed -

III It can be noticed that this feature is closely associated with a regular pattern in the pronunciation, the phonetic vowel being always the strong short vowel except for -AR, -ER, -IR, -OR, -and -UR endings (long vowel), that is excepted if the last consonant is r.

• "/ʊ/" in "put" (alternative short vowel)

It is apparent that this feature has been introduced into the language so as to make the spelling compliant with the principle that single stressed graphic vowels followed by consonant clusters get the strong short pronunciation. While this indicates the pronunciation it also provides a way to distinguish words (furry/ fury, starring/staring, scarring/scaring, sitting/siting, robbed/robed, …). This latter aspect operates as an initial feature (not obtained by inflection) in other pairs: latter/later, matter/mater, comma/coma, rudder/ruder, bitter/biter, chaffer/chafer …

Here is the justification for consonant doubling in the base forms.

Why do we have double consonant letters? This is, for the most part, because originally in English and probably at one stage in French, there was a distinction between short and long consonants. For example, in early Middle English sune ‘son’ was distinct from sunne ‘sun’. When the following consonant was long or doubled, the vowel sound before it was—or became—short, and quite often when the consonant was short or single, the vowel sound before it was—or became—long. When the distinction between long and short consonants ended, during Middle English, the writing of a double, as opposed to a single, consonant became a useful device to show that the preceding vowel was short, and we still retain this convention (albeit inconsistently). For example: bitter versus biter, chaffer versus chafer, etc.

English consonant doubling according to Carney

D1 Double consonant letters do not normally follow long vowels ("write" (but "written" as the i gets the short phonetic vowel),"striding", "gliding", "rhyming", "poked", "stoking", etc.)