Well, the 'a' is never pronounced like the 'a' in 'haunt'. There are a few rules to know how the 'a' is pronounced:
Why? -en is one of several Dutch plurals. So 'man' becomes 'mannen' (extra n added because reasons). English just got a little confused with its plural: it used to be that the vowel changed, but then we borrowed over -s and it's kind of a mess. Just know that in Dutch it's usually a little more regular than in English.
German got double confused on this front:
the men (vowel)
de mannen (-en)
die Männer (vowel + -er)
Anyway, for now: -en = plural. (Not to be confused with en = and, although that might be a good mnemonic.)