At the beginning of English, there was concordance for the second person singular of all verbs in the present tense, as well as in the past of some common verbs. It was usually in the form -est, but -st and t also occurred. Note that this does not affect terminations for other people and numbers. The very irregular verb to be is the only verb with more coherence than this one in the present tense. For example, in Standard English, we can say that I am or that he is, but not « I am » or « he is ». This is because the grammar of language requires that the verb and its subject correspond personally. The pronouns I and him are the first or third person respectively, just as the verb forms are and are. The verb must be chosen in such a way as to have the same person as the subject, unlike the fictitious agreement based on meaning.   For example, in American English, the un expression is treated as a singular for the purposes of the agreement, although it is formally plural.
Compared to English, Latin is an example of a very volatile language.. . .