"Marcus comes from Germany."

Translation: Marcus a Germania venit.

[Ingenti314]

Shouldn't it be Marcus ab Germania venit?

[Moopish]

It's always ab before vowels, but usually a before consonants (though I think you sometimes find ab consonants, someone can correct that if I am mistaken).

[Tristam212765]

When do you use marcum over marcus?

[Moopish]

Marcus is nominative, used as the subject of a sentence, if Marcus is doing an action. Used here since he is the one 'coming'.

Marcus librum habet (Marcus has a book). Marcus is 'doing' the having.

Marcus is accusative, usually the direct object of the verb, used if Marcus has some action done to him.

Livia Marcum videt (Livia sees Marcus). Marcus is the one being seen.

[MathewKeen1]

Do you use "a" when your trying to say comes and "ab" when your trying to say coming? Or is it "a" when its a proper name like Germany and "ab" when its a general name like city or market?

[Moopish]

Mentioned above. It has to do with what kind of letter the word starts with. Ab appears before vowels, like ab urbe. A tends to be used before consonants, like a germania here.