An infinitive is the root word of a verb plus the word ‘to’. Thus an infinitive verb is relatively easy to spot because it is always preceded by the preposition ‘to’. For example, the infinitive of the verb ‘spots’ is ‘to spot’, of ‘ages’ is ‘to age’, of ‘loves’ is ‘to love’, of ‘called’ is ‘to call’, and so forth. Note also that the infinitive of ‘is’ is ‘to be’.
Not all words preceded by the word ‘to’ are verbs or form infinitives. For example, in the sentence, ‘Geraldine gave the ball to me’, the phrase ‘to me’ does not form an infinitive.
A surprisingly common fault in essay writing is what is known as the split infinitive. That is, the ‘to’ and the verb word get separated by other words, usually an adjective or adverb. For example: