verb (singular present am /am, əm/; are /ɑː, ə/; is /ɪz/; plural present are; 1st and 3rd singular past was /wɒz, wəz/; 2nd singular past and plural past were /wəː, wə/; present subjunctive be; past subjunctive were; present participle being; past participle been /biːn, bɪn/)
Old English bēon, an irregular and defective verb, whose full conjugation derives from several originally distinct verbs. The forms am and is are from an Indo-European root shared by Latin sum and est. The forms was and were are from an Indo-European root meaning 'remain'. The forms be and been are from an Indo-European root shared by Latin fui 'I was', fio 'I become', and Greek phuein 'bring forth, cause to grow'. The origin of are is uncertain
For a discussion of whether it is correct to say that must be he at the door and it is I rather than that must be him at the door and it is me, see personal pronoun (usage).