Explanation: an infinitive is the basic form of a verb, without an inflection binding it to a particular subject or tense, it can serve as an adjective, an adverb, or a noun. In the given sentence we can see an example of an infinitive in the words "to follow" and, in this case, it are acting as a noun, specifically as the direct object of the sentence (it answers the question "what is his greatest ambition?").
The verb in the sentence is, to follow (his father's footsteps). This verb is used as an infinitive. An infinitive is a word in the verb form that functions as a noun, adjective or adverb. Infinitives are generally introduced by the word "to".
