Which type of verb typically includes the word "to"?

a. Past participle

b. Present participle

c. Past tense

d. Infinitive

The answer is D. A verb that starts with "to" is in an infinitive verb form. It should be noted that a verb in this form does not function as a verb instead it functions as another part of speech. Infinitives can function as adverbs, adjectives or as nouns.