can I say " I used to smoke for 5 years "?
is it correct?
And if no, which tense should I use?
Should I say "I had smoked for 5 years?"
Thanks a lot.
I agree with @FumbleFingers. "I smoked for 5 years" is correct, and "I used to smoke for (time period)" is not. "Used to" is implied when you use the past tense - i.e., it is understood that you no longer smoke. Like @FumbleFingers says, "for (time period)" only makes sense if you're describing how long each discrete instance of smoking lasted. Not the case here.
(EDIT - the following paragraph is just intended to show the different meanings indicated by the different tenses - I still think the simple past is correct in this case!)
"I have smoked for 5 years" (present perfect) would tell me that you still smoke now after doing so for 5 years. In other words, using this tense would mean "I used to smoke and continue to this day". This would be in contrast to "I smoked for 5 years" (again, simple past). The connotation of the latter is "I used to smoke but no longer do." I think this is what you're getting at, and therefore you should use the simple past, "I smoked...", as explained in the first paragraph.