ich bin = I am
du bist = you are (informal, addressing one person)
er/sie/es ist = he/she/it is
wir sind = we are
ihr seid = you are (informal, addressing more than one person)
sie sind = they are
Why is it "ein" instead of "einen"?
The verb sein takes Nominativ. Grammatically ein Mann is part of the predicate: ich - subject, bin ein Mann - predicate.
You can see them as two separate sentences. It's "ein Mann" because it belongs to the subject "ich".
Why do you use 'ein' for 'girl', while 'eine' for 'woman'? I feel so confused about 'ein','eine' and 'einen'.
It's because "Mädchen" is Neuter, not feminine.
Man v Mann?
Mann = Man and Man = one (as in "one must be careful")
Why isn't i of 'ich' capitalised ?
Because we don't capitalize our "I". In German it's always "ich". But we have a sie (she) and a Sie (formal you).