Uses of the to-infinitive
To-infinitive as the subject
Look at the following sentences:
To err is human.
To find fault with others is easy.
To quit now would be foolish.
You will have noticed that in all the three sentences given above the subjects are infinitives (to err, to find fault with and to quit). Thus the to-infinitive can be used as a noun to form the subject of a sentence.
Notes: However, it should be noted that it is more common to write these sentences with a preparatory it as the provisional subject.
It is human to err.
It is easy to find fault with others.
It would be foolish to quit now.
To infinitive as the object
Read the following sentences:
John likes to watch action movies.
Alice offered to quit the job.
Peter decided to contest the election.
John likes … what? To watch
Alice offered … what? To quit
Peter decided … what? To contest
Here the to-infinitives to watch, to quit and to contest are the objects of the verbs likes, offered and decided. To-infinitives are thus used as noun equivalents to form the objects of transitive verbs.
To-infinitives as subject complements
Now read the following sentences:
Your duty is to finish the job on time.
The mistake she made was to invite her aunt.
Here the to-infinitives to finish and to invite are the complements of the subjects your duty and the mistake respectively.