Middle English was spoken from 1150 to 1500. Unlike in contemporary English, the verbs of Middle English were highly conjugated. The conjugation of Middle English verbs was similar to that of German and Dutch.
Here is the present tense conjugation of the verb sing. It is important to note that the plural forms varied according to dialect. The following is a southern dialect:
I singe (I sing)
thou singest (you sing)
he/she singeth (he/she sings)
we singen (we sing)
ye singen (you sing)
they singen (they sing)
The Middle English verbs for first and third person plural are the same as in German. In Middle English we see four different verb conjugations, but in contemporary English we see only two.
Now let's look at the verb have.
I have (I have)
thou hast (you have)
he/she hath (he/she has)
we haven (we have)
ye haven (you have)
they haven (they have)
The Middle English verb for second person singular is the same as in German. Again we notice that Middle English has four different verb conjugations, but contemporary English has only two.
The verbs of Middle English were more highly conjugated than those of modern English. It is clear that modern English has simplified the conjugation of verbs. This isn't surprising because simplification is a common trend in all languages.