A defective verb, as previously stated, is one that lacks some of its principal parts.
Beware is defective; it has no participles.
Ought is used only in the present and past tenses, and has the same form in both. It is in the present tense when followed by the present infinitive:
I ought to go.
It is in the past tense when followed by the present perfect infinitive:
I ought to have gone.
Quoth I. Quoth he.
It means said, and is now rarely used.
To wit, meaning to know, is now used only in the present infinitive, in the sense of namely, that is to say. The form wot is found in old writers:
I wot God wot.