With masculine nouns like Mann, the indefinite article einen is used for accusative case, ein for nominative.
When you have a predicate noun (like Mann in this sentence), you use nominative. Here is some further reading... Pay attention to the second paragraph as it shows you which verbs to look out for... http://coerll.utexas.edu/gg/gr/cas_02.html

Ein use for men and eine use for women

No. Same verb, different conjugation depending on subject. You are = du bist. He is = er ist.
Here's the conjugation table for "sein" (to be):
http://canoo.net/inflection/sein:V:sein

Both bist and ist is the verb conjugation for the irregular verb sein (to be). Here is a conjugation chart for it: ich - bin = I am du - bist = you are er - ist = he is sie - ist = she is es - ist = it is wir - sind = we are sie - sind = they are Sie - sind = formal you are ihr - seid = ya'll are So basically they are the same word conjugated to different pronouns. And no verbs do not have masculine, feminine, or neuter forms their forms change based on the noun/pronoun.

Er = he
Sie = You (formal), or, if at the start of a sentence Sie could be you (formal), she, they, depending on verbs
sie = they, she, depending on verbs

Here's a list of personal pronouns:
http://en.wikipedia.org/wiki/German_declension#Personal_pronouns.5B3.5D

Hope that helps!

Mann = man, an adult male human, opposite of a woman, can also be used for "husband"

Mensch = person (male or female), man (in the sense of mankind), human being