"You will finish writing your composition later."
Translation:Tu finiras d'écrire ta rédaction plus tard.
Finir is one of the French verbs that requires a preposition: https://www.thoughtco.com/french-verbs-with-prepositions-p2-1364548
English is inherently fussier than French about placement of words; placing 'later' anywhere but at the beginning or end of a sentence like this wouldn't look right. So the fact that it is in a particular place in English doesn't necessarily mean that it has to be (or is even allowed to be) in the same place in French. As I understand it, French would allow one more option: placement immediately after the verb being modified. So you should be allowed to say 'tu finiras d'écrire plus tard ta rédaction. It doesn't belong after 'finira' because what is happening later is not simply the finishing but the finishing of the writing. Also, note that strictly speaking, the verb isn't 'finir' but 'finir de', and I don't think you can separate the two parts of the verb.