An infinitive is a simple verb combined with ‘to.’ For example: to sleep, to run, to fly, to hide
Remember this formula: Infinitive = to + verb
Infinitives can function as subjects, objects, adjectives, and adverbs. For example:
To wait for an answer seemed like a waste of time. (To wait is the subject of the sentence.)
I can’t stand it; I refuse to look. (To look is the direct object of the sentence.)
If you get bored easily, bring a book to read. (To read is an adjective modifying book.)
Martin planned this event to show Diane how much he cares about her. (To show is an adverb explaining why Martin planned the event.)
You may have heard that ‘splitting infinitives’ is the worst of all grammar crimes. What does this mean? Splitting an infinitive simply means to add other words between the ‘to’ and the verb in an infinitive. For example: “I plan to really enjoy the party.” (The word ‘really’ is in the middle of the infinitive.)
Split infinitives are still frowned upon in formal writing situations. For this reason, split infinitives are best left out of a sentence. However, it is not wise to mangle a sentence simply to avoid splitting an infinitive.
To learn more about verbs read this blog post about verb tenses.