General things to remember about moda lverbs
1. They stay the same in every form. We never add -s.
He should go.
She would love it.
It will be great.
2. After the modal verb we always use the base form of the main verb.
I can drive.
You could see.
She must save all her salary.
This is true also in the past when we use have after the modal verb.
They must have gone.
I could have won.
They should have worked harder.
And also when in the present continuous when we use be after the modal verb.
He should be studying.
They must be watching tv.
They shouldn't be sleeping.
3. When we make a question, we invert the modal verb and the subject.
Can you speak French?
Should he be here?
Shall we go?
Modal verbs are used in the following ways:
English Grammar Reference
Ability (can, could)
Permission (can, could, may)
Obligation and advice (must, should, ought to)
Possibility / Probability (must, might, could, may, can't)
Suggestions (should, ought to, shall)
First, Second and Third conditionals (will, would)