The verb "spend", meaning to use time, is a transitive verb i.e. it needs an object to make sense. You can say we spend an evening, not an evening spend/spends. However, you can say an evening is spent (by us) that's in the passive.

So you can say an evening that is spent with friends or an evening spent with friends. The relative clause "that is spent" has been reduced to "spent".

Further, the sentence will also be grammatical if you say "..... an evening (that) we spend with friends".

For me as a native speaker with imperfect grammar, I suggest the following: Both versions are correct. The first is a general statement that you regularly write for many hours (I spend hours writing [something]). The second version is in the past tense (I spent hours writing [something]). Therefore it has already finished. Finally, I would use the gerund (-ing) rather than the infinitive (to write) in both of these sentences unless I were to say something a bit different e.g. "It took me hours to write that" but.this is a very different sentence structure. I don't fully understand the grammar behind it, perhaps your teacher can help. Otherwise you might just be able to learn it as a phrase and work from there. Therefore I would actually say something like: I spend hours writing my English homework. (A regular activity) I spent hours writing my English homework. (A past activity) It took me hours to write my English homework. (Past tense again) There are many other options depending on the tense that you want to use. Good luck!

spent

I've never heard it used this way before, but I wouldn't be surprised if some native speaker used it like this, since the "nouning of verbs" has become quite common.