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.
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.