a) Ability: "can", "could", "be able to"
a.1. "can" / "could"
We use "can" ("could" en el pasado) to say that something is possible or to say that someone knows how or has the ability to do something.
can (could) + infinitive
- I can see the lake from my window
- Can you speak Japanese?
- We can't go to Paris this weekend
"Can" can only be used in the Present and the Past. It cannot be used in the present perfect. Can does not have an infinitive.
a.2. "be able to"
We can use "be able to" instead of can.
be able to + infinitive
- I haven't been able to do my homework. I will do it tomorrow
- Are you able to play the piano?
b) Obligation: "must", "have to", "have got to"
We use "must" to say that it is necessary for someone to do something
- I must go to work today
- They mustn't smoke in this building
- We use "must" when we feel that something is true
- You have been working all day. You must be tired
- Manuel has been translating all day. He must be bored
must + have + past participle
- I have lost my mobile phone. I must have left it in the library
b.2. "have to + infinitive" / "have got to + infinitive"
We use "have to" / "have got to" when you talk about external obligation, such as when a law or someone says that something is necessary or important.
- Tonight we have to go to swimming practice (if we don't we won't be in the team)
- They have got to go to school on Friday
c.1. "may" / "might"
may (might) + infinitive
- They may go to the party on Saturday
- I might wear a dress
- We may not be able to go to school this week
- She might not (mightn't) play tennis at Wimbledon
We use "may" or "might" to talk about possible actions in the future.
"May" and "might" are usually interchangeable. However, we use "might" (not "may") when the action is not real.
If I knew your Mum better, I might invite her over for a cup of tea (The situation is not real because I don't know the Mum better, so I'm not going to invite her. May is not possible in this example).
should (ought to) + infinitive
e) Lack of obligation / responsibility
don't have to (needn't) + infinitive