Can - Can't (=cannot)

• to talk about ability

  "John can speak French"

• To make requests

  "Can you help me?"

• To talk about possibility

  "I can help you tomorrow"

• To ask for or giv permission

  "Leave, you can go"

Could - Couldn't

• To talk about general abilities in the PAST

  "I could run faster when I was younger"

• To make requests

  "Could you please stop talking so loud?"

Can ability in the present and the future, whereas for the past we use COULD

Be albe to

positive: subject + to be (am-is-are) + able + to infinitive

negative: subject + am not - isn't - aren't + able + to infinitive

question: am - is - are + subject + able + to infinitive

"John is able to speak French"

"I wasn't able to swim"

"Will she be able to take the test again?"

Must

positive: speaker's decision

negative: prohibition