'Should do' is used to talk about general situations or the future. It is used to give advice or sometimes to talk about probability. e.g. "People should not eat too much red meat." "Children should get more exercise." "You should go to the party. it'll be fun." "I should arrive on time if the traffic isn't heavy." "should have done" is used to give advice about the past e.g. I want to be a doctor. However, I didn't study hard enough at school so now I can't be a doctor. You can say to me, "You should have studied harder at school." e.g. I have a headache now because I drank too much last night. You can say, "You shouldn't have drunk so much last night." (It is actually not very useful because it is about a situation I cannot change now ;-))

All you need to do is to pay a little more attention to the class, if you are still a highschooler or even younger. It's shame I didn't and I was stupid, and I had to figure them on my own. Claire is right about the case; my explanation has some mistake, what I mentioned formerly was for sentence like this: 1> If I had paid attention to school, I should have been decent at English now. This is to mean, in the past I didn't work hard at school; 2> If I had paid attention to school, I should have got my college degree when I was 18 (past tense). This is to mean, what you were looking forward to should be/'ve been achieved/happened in the past. I think the grammar here is a little bit confusing. In a cluase, better be: I told you that you should do that. This sounds like you didn't do that. Non-clause: You should have done that. Means you clearly didn't do that.

All you need to do is to pay a little more attention to the class, if you are still a highschooler or even younger. It's shame I didn't and I was stupid, and I had to figure them on my own. Claire is right about the case; my explanation has some mistake, what I mentioned formerly was for sentence like this: 1> If I had paid attention to school, I should have been decent at English now. This is to mean, in the past I didn't work hard at school; 2> If I had paid attention to school, I should have got my college degree when I was 18 (past tense). This is to mean, what you were looking forward to should be/'ve been achieved/happened in the past. I think the grammar here is a little bit confusing. In a cluase, better be: I told you that you should do that. This sounds like you didn't do that. Non-clause: You should have done that. Means you clearly didn't do that.

@ 一 ：Thanks, haha.....your English is pretty good ! ^^

First off "should have done" refers to the past (perfect) tense, you regret that you didn't do something you were supposed to do. I should have told you that, means I needed to, it was necessary for me to tell you that but in fact I didn't tell you that. OK, then the point of the "should've done" is, it somehow indicates "what you should have done" had some influence on something that already happened by now. I'm sorry I couldn't go there, but you did go, I should've told you that I couldn't. Then, regarding "should go", aside from its usual usage "you should work hard everyday", it could be a blame on what you did not do, that has a usually negative influence on something right now is going on/happens. I thought you should do that, see, now you are wasting your time. To be not very strict, this almost equals to saying "I thought you should have done that, see, you are ...ing now".