- In Duch they are called 'Hulpwerkwoorden'.
- Modal verbs never stand alone.
- Modal verbs are placed before the main verb (hoofdwerkwoord) to change the meaning of the sentence. That's also why they never stand alone ;).
For example:
I swim every Tuesday. --> Fact
I can swim every Tuesday. --> Possibility (hypothetical)
Here 'can' is used as a modal verb. 'Swim' is the main verb.