am (v.)youdaoicibaDictYouDict[am 词源字典]
Old English eom "to be, to remain," (Mercian eam, Northumbrian am), from PIE *esmi- (cognates: Old Norse emi, Gothic im, Hittite esmi, Old Church Slavonic jesmi, Lithuanian esmi), from root *es-, the S-ROOT, which also yielded Greek esti-, Latin est, Sanskrit as-, and German ist.

In Old English it existed only in present tense, all other forms being expressed in the W-BASE (see were, was). This cooperative verb is sometimes referred to by linguists as *es-*wes-. Until the distinction broke down 13c., *es-*wes- tended to express "existence," with beon meaning something closer to "come to be" (see be).

Old English am had two plural forms: 1. sind/sindon, sie and 2. earon/aron The s- form (also used in the subjunctive) fell from use in English in the early 13c. (though it continues in German sind, the 3rd person plural of "to be") and was replaced by forms of be, but aron (aren, arn, are, from Proto-Germanic *ar-, probably a variant of PIE root *es-) continued, and as am and be merged it encroached on some uses that previously had belonged to be. By the early 1500s it had established its place in standard English. Art became archaic in the 1800s.[am etymology, am origin, 英语词源]