It is traditional to use modal verbs in English language essay writing. So it is very useful to be able to use them correctly.
I found this website has a simple, yet easy to understand explanation.
The key point is that after a modal verb, the simple form of the verb is used.
I can do the washing tomorrow.
After 'can' the verb is 'do' not 'to do'
I should work on my assignment tonight.
After 'should' comes 'work' not 'to work.'
Hope that helps.