Unit 1: Past simple vs. Present perfect simple.
1. - We use the past simple to talk about complete events which are finished, or before “now”, the moment of speaking. I called you yesterday. Where were you? We didn’t have computers when I was born.
2. - We use the present perfect to connect the past and “now”, the moment of speaking. We’ve called you three times today. Where have you been? , We’ve lived in the same house all our lives.
3. – Use the past simple with minutes ago, yesterday, last week, when I was….etc. We often use for, since, just, already, yet, even and never with the present perfect. They went out a few minutes ago. , I saw that movie yesterday. , I met her boyfriend last weekend. , We moved there when I was young. , …ver más…

Time conjunctions: as/then/as soon as.
Other time words that we use with the past simple are then and as soon as. We can also use as with the same meaning as while. As soon as got home, I turned the TV on to see the big game.
Past simple vs. past perfect.
1.- We use the past simple to talk about an event that happened at an specific time in the past. We use the past perfect when we need to emphasise that one event happened before another. The match had started when we got there.
2.- Sometimes it is necessary to use the past perfect to make the meaning clear. She´d left when I got there. (I didn’t see her).
3.- It is not necessary to use the past perfect when before or after is used. She left before I got there.
Unit 3:
Present perfect simple vs. present perfect continuous.
1.- We use the present perfect simple to emphasise the result or completion of an activity. I´ve copied that CD you asked me for. Here it is. We use the present perfect continuous to emphasise the activity, not the result or completion of the activity. I’ve been copying CD´s all the morning. Great fun!
2.- We use the present perfect simple to emphasise “how many”. I´ve done ten exercises this morning. We use the present perfect continuous to emphasise “how long”. I´ve been doing exercises for hours.
Had better/should/ought to.
We use should or ought to to give advice, or say what we think is a good or bad idea.