USES "CAN"

We use 'can' to talk about 'possibility'.

Can you do that?
I can't manage to do that.
You can leave your car in that parking space.
You cannot smoke in here.
Notice that there are two negative forms: 'can't' and 'cannot'. These mean exactly the same thing. When we are speaking, we usually say 'can't'.

We use 'can' to talk about 'ability'.

I can speak French.
I can't drive.
We use 'can' to ask for and give permission. (We also use 'may' for this but is more formal and much less common.)

Can I speak to you or are you too busy?
You can use my phone.
You can't come in.
We use 'can' in offers, requests and instructions.

Can I help?
Can you give me a hand?
When you finish that, you can take out the garbage.
We use 'can' with 'see' 'hear' 'feel' 'smell' 'taste' to talk about something which is happening now . (Where you would use the present continuous with most other verbs.)

I can smell something burning.
Can you hear that noise?
I can't see anything.
We can use 'can't' for deduction. The opposite of 'can't' in this context is 'must'..

He was in London one hour ago when I spoke to him. He can't be here yet.
You can't be hungry. You've just eaten.
You must be hungry. You haven't eaten anything all day.