"You could do the task" expresses the possibility that someone could, in the near or far future, do a certain task. "You could have done the task" says that someone could have done something (in the past) that they didn't do. Hope this helps!

Wow,This is really obvious and clear. Thanks Mr Milton. :)

"Could have" is past tense; "could" is present tense, even though you probably learned that "could" is the past tense of "can". It's not really the in past tense in that sentence, it's in the subjunctive mode. We use the subjunctive when we want to be more polite; therefore "you could do the task" is more polite than "you can do the task". Both of these phrases are actually used to talk of the present {tense}. Another way of stating "could have" is "you were able to (get the task done}"