Can : This word can tell the ability, requst and permission.
Ex: Your sister can speak English very well. (ability)
You can take my care. (permission )
Can he pass me the salt? (requst)
We can use "can" on positive, negative and interrogative.
Ex: The tempurature can sometimes reach 40c in April. (positive)
I can't see you today but I could see you tommorrow. (negative)
Can you speak French? (interrogative)
Remember: We can use "Could" for past of "can" and use it for more polite way.
Ex: I could swim. (past of can)
Could you turn on th light? (polite way)