How do I say "did you watch the football?"
"你看足球比赛吗？" = "do you watch football match?"
"你看了足球比赛吗？" = "have you watched the football match? " (看 is the verb, 了 is the verb particle indicates 'completion of an action-- 足球比赛 is the object that was being watched)
"你看足球比赛了吗？" put the object '足球比赛' between '看' and its verb particle '了'. that make '看足球比赛' a verb phrase


If you want to ask if someone watched the football match, #2 and #3 are both acceptable.