The modal verbs
They express things like ability, permission, possibility, obligation etc. Modal verbs only have one form. They do not take -s in the simple present and they do not have a past simple or past participle form. However, some modal verbs have alternative forms that allow us to express the same ideas in different tenses.
To find the correct form, you can use an online database of the irregular verbs.
For example: irregular verb CAN