The infinitive form of 'instruct' is simply 'to instruct'. All infinitives forms are preceded by 'to' in English.
The infinitive form of "am" is "to be," the infinitive form of "is" is "to be," and the infinitive form of "was" is "to be."
The infinitive form of had and has is to have.
The infinitive form of "was" and "were" is "to be." The infinitive form of "are" is also "to be."
The infinitive form of "are" is "to be."
The infinitive form of "has" is "to have." "Have" is the base form of the verb, and when used with "to" before it, it becomes the infinitive form.
The infinitive form of "is am are" is "to be."
The verb form of instruction is "instruct."
The infinitive form of "did" is "to do".
Infinitive is the basic form of a verb. "Let" is the infinitive in this case.
The infinitive form of "sit" is "to sit."
The infinitive form of "should" is "to should."
The infinitive form of "think" is "to think."