What is the sign of infinitives?

Infinitives don't have a specific "sign" in the way that other parts of speech do. Instead, they're identified by their form, which is:

* "to" + the base form of a verb:

For example:

* to run

* to eat

* to sleep

* to sing

Key things to remember about infinitives:

* They can act as nouns, adjectives, or adverbs.

* They are not verbs in their own right.

* They can be used in phrases with other words.

Let me know if you'd like to see some examples of how infinitives function in sentences!