The fourth principal part of the verb (e.g. if you look up
amo in a dictionary, you'll find
amo, amare, amavi, amatus; that fourth word
amatus is the 4th principal part of
amo) is one possible participle that can be used in the ablative absolute. This participle is declined like
bonus, -a, -um.
Since this is the past-passive participle, you use it in the "with (noun) having been (verb)" pattern, e.g.
militibus dimissis - "with the soldiers having been dismissed".
miles, -itis - "soldier" is a 3rd decl. noun, so its ablative plural form ends in -ibus. The verb here is
dimitto - "dismiss"; look it up to confirm the 4th principal part is
dimissus, and note the masculine abliative plural ending is -is (like any adjective,
dimissus must agree with the noun it is modifying--
militibus--in gender, number, and case.
Master this before worrying about the present participle and phrases like
viro stante...good luck