What happened to the “‑est” and “‑eth” verb suffixes in English?

What happened to them, and how were they once used? Straining my mind to sound archaic, I came up with the following:

Dost thou thinkest thou can escape thy sins?

and

Bringeth me mine armor and favorite sword.

I’d like to use these suffixes intelligently, so my questions are: how are ‑est and ‑eth properly appellated in conjugations, and when and why did they disappear?

Verb paradigm in King James English for think

   Singular             Plural
   --------------------------
1  (I)    think         (we)   think
2  (thou) thinkest      (you)  think
3  (he)   thinketh      (they) think

Imperative: think
Infinitive: (to) think

These unfamiliar suffixes are applied in the same context that the -s suffix is applied in Modern English; for example:

• He thinks.

• Thou thinkest.

  ---

• He shall go. (no -s suffix on go)

• Thou shalt go. (irregular verb form for shall; but again, no suffix on go)

During the Early Modern English period, the 2nd person singular suffix disappeared and the -th suffix in the third person was replaced by another suffix, -s, which spread from dialects in the northern parts of the country. Other conjugations, such as -e in the first person singular from Middle English, had already been lost.

This sort of change is known as paradigm leveling. There is no particular reason per se that this kind of change happens, but it is not uncommon in the languages of the world.

It's basically a feature of being a Germanic language. In German, the second person takes -st, e.g., "Ich bin, du bist, Ich kenne, du kennst" (I am, you are, I know, you know).

The old English forms follow the same pattern, as Kosmonaut shows above. This is the easiest way to spot "fake Elizabethan" language in books and movies. "Wouldst thou?" is fine, "wouldst I?" don't fly.

-Est was dropped when the uniquely singular "thee/thou/thy" fell out of use. -Eth was eventually replaced by -s or -es; the point being that language changes over time and tends to move toward simpler forms.

In English, as in other languages, we employ "First Person" (the one speaking), "Second Person" (the one being spoken to), and "Third Person" (the one being spoken about). There is singular (I speak) and plural (we speak).

Using French as an example, I can say tu parles to my friends (Second Person Singular), but to show respect I would say vous parlez, whether to one of you with whom I'm not familiar or to more than one of you. So vous is the plural as well as the formal form of tu. The same in English. You is the plural of thou.

Other languages use she and the Third Person to show respect, since such words as grace and majesty are feminine (Sie in German and Lei in Italian). Still others use a word formed from Your Grace to show respect (Vuestra Merced in Spanish becomes Usted after Vuestra being abbreviated as "Vst" which became "Ust" and combined with "-ed" from merced. The Portuguese você has a similar construction coming from vossa mercê, or Your Grace.) In some Spanish-speaking countries, the formal Usted (which, keep in mind, takes the Third Person form) replaced the Second Person tú.

This is what happened in English: the formal use of you, even in the singular sense, replaced thou almost everywhere except, for example, among the Amish: thou speakest and you speak. The same for their object pronouns thee and ye which were replaced by the subject pronoun you. So that's what happened to thou and its suffix "-est."

As for the ending "-(e)th," that was replaced over time by "-(e)s": he maketh becomes he makes.