An infinitive is the "to" form of the verb. The infinitive form of "learn" is "to learn." You can also use an infinitive as the subject, the complement, or the object of a sentence.
Examples:
1. To learn is important. ( subject of sentence )
2. The most important thing is to learn. ( complement of sentence )
3. He wants to learn. ( object of sentence )

Infinitives can be made negative by adding "not."
Examples:
1.I decided not to go.
2.The most important thing is not to give up.