a question

[zzzoeee]

Es ist ein Mann,why not einen?

[fioccodineve]

The "Discuss sentence" button that appears after you answer a question can be use for this type of discussion.

[liamtbrand]

Like @fioccodineve said, and, because Mann is masculine.

[Tomk123]

The verb 'ist' is an equative verb which means that its subject and its complement refer to the same thing. Therefore the complement is not having something done to it, as with the verb 'hit', so it takes the nominative case the same as the subject. Although, in English this rule is not always followed. No one says. 'it is I' nowadays, but 'it's me'.