Defective verbs are those in which some of the prin­cipal parts are wanting. Defective verbs include:

• may
• can
• shall
• will

These verbs have the past tense forms but do not have participle forms.

The following verbs are also included in the defective verbs:

• must
• ought

These have neither past tense forms nor participle forms.

When must refers to past time, it is used in its present perfect tense form.

For example

He must have left.

When ought refers to past time, it is followed by the perfect infinitive.

For example

He ought to have written.