Neither of your alternatives really work properly. Try these instead: "We can WATCH TV when we have finished our homework" "We WILL BE able to WATCH TV when we finish our homework" In English, if you 'see' a TV, you are looking at the physical object which you call a television, (I see a TV in the corner of the room).If you are looking at the picture on a TV, you are 'watching' it (last night I watched TV until midnight), or you are 'watching' something 'on' TV.

No ! The first sentence is more correct. Your sentence means that till you finish your homework, you are incapable (mostly physically) of watching the television. We can watch or We may watch is better in this case. Also in case someone has put a condition in front of you asking you to finish your homework as a condition of being allowed to watch the tv, you may say "We will be able to watch TV when we finish our homework"