An infinitive will almost always begin with 'to' followed by the simple form of the verb as:
To + Verb = Infinitive
Infinitives can be used as nouns, adjectives, or adverbs.
The simple trick in recognizing infinitives is to look at the word 'to'. To run, to sit, to cry, to jump, to drink, to read, to eat- all of these are infinitives.
Infinitives can be used in the following forms:
In the above mentioned examples: to play, to dance and to sing are the subjects of the sentence and this gives us a hint that it is being used as a noun.
The most important thing to keep in mind while using infinitives is the use of split infinitives. The general rule is that no word should separate the 'to' of an infinitive from the simple form of the verb that follows. If a word comes between these two components, it results in a split infinitive.
Sara hopes to quickly finish her chemistry homework so that she can return to the more interesting Stephen King novel she had to abandon. (Incorrect)
Sara hopes to finish her chemistry homework quickly so that she can return to the more interesting Stephen King novel she had to abandon. (Correct)