In general, modal verbs syntax is the same for all:
Subject + [can/could+ verb(simple form)] + Complement
Subject + [can't/couldn't+ verb(simple form)] + Complement
Question word + [Can/Could + verb (simple form)] + Complement
A) Ability ( Can : present , Could : past )
To express an ability we use can/could
- Sheila can play the guitar very well
- When I was a child I could play all day.
- Greg can't cook at all.
- Can you speak French?
- Pete could sing when he was in college.
Remember that when you talk about ability, can is used for present and could or past abilities
B)Possibility: (Can = Could)
When you express possibility can = could.
- I think it (can/could) rain.
- I (can/could) buy a new car because I have a good job.
C) Other uses:
- Can - to ask permission: Can I go to the Bathroom? (May : formal)
- Can / Could you bring me some Coffee? : Ask Favor (Pedir favores)