"It's a very interesting movie. Have you seen it?" Present perfect is more common because you are interested in knowing if the activity took place at all (not when, where, with whom, etc.). And it might be important for present discussion. Tom: "Have you seen Lady Gaga in concert?" Fred: "No. I haven't." Tom: "Well, here's a DVD of her last concert. Take it home and watch it." The next day: Tom: "Did you watch the DVD of the Lady Gaga concert?" Fred: "No. I didn't have time last night." "Watch" is more about the act of viewing. "See" is more about watching and understanding. 2. "I had been smoking for four years. Then I decided to give it up." Past perfect progressive lets us know that the period of time ended in the past and more time (four years) has passed since that time.

1. Neither is correct. "This a A very interesting movie...." 2. No, you can't say that. "Used to" and "have done" are not interchangeable.

1. "This (It) is A very interesting movie! Have you seen (watched) it?" - In this case, I bet you most people are going to use "seen" but "watched" is not incorrect. "This (It) is A very interesting movie! Did you see (watch) it?" - Both of them are correct. You can interchange them. 2. A situation: I used to smoke. I started smoking when I was 17. But 4 years ago I quit smoking. Will it be correct if I say NOW "I have been smoking since I was 17"? - NO. This implies that you STILL smoke. "I had been smoking since I was 17." This will imply that you no longer smoke, but no one will know how long ago. If you want to say "4 years ago" then maybe "I had been smoking since I was 17, but I quit 4 years ago."