Use "you had" when talking about the past. Use "you have had" when talking about the present (ie. the present situation as a result of the past). To pilfer Ian's example, "You've had ten beers" means "You are very drunk now". If you say "you had ten beers", then that tells me nothing about the present.

Hi, My first attempt at answering these questions! I'll try to explain with examples. "When I saw you at the party last week, you had no idea that I was going to bring Jessica as well" "I think you should stop drinking, You have (You've) had 10 beers already" Hope this helps?!