An infinitive form, that uses the plain form of the verb and the preposition to, is used for verbal clauses that are syntactically subordinate to a finite verbal clause. Finite verbal clauses are those that are formed around a verb in the present or preterit form. In clauses with auxiliary verbs they are the finite verbs and the main verb is treated as a subordinate clause. For example, he has to go where only the auxiliary verbhave is inflected for time and the main verb to go is in the infinitive, or in a complement clause such as I saw him leave, where the main verb is to seewhich is in a preterite form, and leave is in the infinitive.