Those are which do not have all forms required to create sentences in different tenses. See explanation for examples.
Some English verbs are missing forms used to create sentences in diifferent tenses.
- the verb can is such a verb because although it has got Simple form could, it does not have Past Participle form used to create Perfect tenses.
If you want to create Perfect tense sentences you have to use be able to form which has all possible forms.
- the verb must has only form.
If you want to create a sentence in any other tense you have to use its equivalent: have to.