We use 'can' to talk about 'possibility'.
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'.
We use 'can' to ask for and give permission. (We also use 'may' for this but is more formal and much less common.)
We use 'can' in offers, requests and instructions.
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.)
We can use 'can't' for deduction. The opposite of 'can't' in this context is 'must'.