When you have a singular noun as subject, a singular verb follows. However, the pronouns "I" and "you" are singular but singular verbs do not follow after them. Does anyone know something about this reason or historical explanations?
For example, you say "I live" instead of saying "I lives".
As Mari-Lou has said in her comment, the first person singular of be is am in the present tense and was in the past tense. Neither is ever used as a plural in Standard English. The second person singular in the present tense is are and were in the past tense. Both are also used for all other numbers and persons except the first and third.
The reason English has such inconsistent forms of the verb be is that Old English had two equivalent verbs, beon and wesan, from different roots. The first person singular of the present tense of beon was eom, which gives us today’s am. Was is from the past tense of wesan, which had wæs in the first person singular. The origins of are are a little less certain, but the past tense of wesan was wære in the second person singular and wæron in the second person plural.
EDIT
I’m sorry, I got distracted by the verb be.
Verbs like live, which are regular in Modern English, had several inflections in Old and Middle English. The first person singular in the present tense was the base form with an optional -e ending, the second person singular had the ending -est, and the third person singular had the ending -eth. All persons in the plural had the ending -e(n). All these forms were simplified after the Middle English period, until now we have only the two forms in the present tense.
I haven’t researched the history of the transformation of -eth to -s, but I would guess that it came about through phonetic change. Others may be able to give a detailed explanation, but the history of English verb forms is complex.
In regular verbs, it’s the third person singular which changes in the present tense. Just because “I live” is the same as “we live,” it does not mean that live is a plural form when used with I. It’s a singular form which happens to be the same as the plural form.
The third person singular is the exception.
Your question implies some a priori reason to expect nouns and verbs to "adhere to the same system" in the first place with regard to plural marking (or presumably, by extension, marking of other features). But it's not clear, looking across languages generally, that there's much pressure for languages to evolve in that way: there are many languages with completely different morphology for nouns vs verbs.
Historically, you could point to features that have provided a "seed" for the -s in the nouns and verbs of English. But it's difficult to pinpoint a precise reason why -s was what "stuck" out of other possible courses of evolution. For example, many masculine nouns in Old English had an -(e)s ending in the plural. But many other nouns didn't: an -an ending (cf the -en of "oxen" today) was actually much more typical overall and a person in 1200 trying to predict the future might equally have thought that -en would prevail. In the verb system, there were various endings that had -s in, but equally you might have predicted that -(e)th would survive rather than -s.
But the historical details don't particularly matter anyway: in Darwinian terms, if there was "selective pressure" for the language to evolve to have "the same system" for nouns and verbs, then the language could essentially have evolved in that way whatever the historical "starting point" (there's really no magical "starting point" of course: languages are continually evolving from their previous state). But there just doesn't seem to be much pressure for languages to evolve in that way: the pressure seems to be rather for verbs vs nouns to have different systems.
