All of the different forms of "to be" come from different words in old languages. We get "am" and "is" from Latin, but I am not sure about the other tenses. Google says that all of the others are a collection of words "Indo-European," which indicates that it is some language from ancient times.
As for answering questions with a presumed answer, one of the best things to do is to restate the verb when you answer. "Didn't you agree with him?" "Yes, I agreed with him." This way, it is clear to everyone.
As for answering questions with a presumed answer, one of the best things to do is to restate the verb when you answer. "Didn't you agree with him?" "Yes, I agreed with him." This way, it is clear to everyone.