We use modals to show how probable something is, we also use them when talking about ability, asking permission making requests and offers.
We can divide the modals into different groups, like this:
First, they can be used when we want to say how sure we are that something happened / is happening / will happen. We often call these 'modals of deduction' or 'speculation' or 'certainty' or 'probability'.
We use 'can' and 'could' to talk about a skill or ability.
Obligation and Advice
We can use verbs such as 'must' or 'should' to say when something is necessary or unnecessary, or to give advice.
We can use verbs such as 'can', 'could' and 'may' to ask for and give permission.
We can use 'will' and 'would' to talk about habits.
The past modals are 'could have + past participle', 'should have + past participle' and 'would have + past participle'