Modal verbos do not take 's' un the third personas :
Ej: he can speak chinese
     She should be here by 9:00

You use not to make modal  verbs negative even in simple present and simple past

Ej: he should not be late
    They might not  come  to the party

Many  modal verbs cannot be used in the past tenses or the future tenses.

Ej: he  will can go  with us (not correr )
     She musted study very hard (not correr).

Common modal verbs.

Can        ought
Could.    Shall
May       should
Might.  Will
Must.    Would