The infinitive of a verb is the verb in its basic form. It is the part that you would find in the dictionary if you were to look up any given verb.
The infinitive of a verb is usually, but not always, preceded by the word ‘to’ (to shout, to climb, to cheat, to run, etc.). ‘To’ is not a preposition here, it merely indicates that the verb is being used in its infinitive form.
- Mum asked me to run to the shops for some eggs.
- My sister likes to help in the kitchen.
- I was lucky to see a fox in my garden
In terms of grammar, the infinitive is regarded as the name of the verb. so for instance you might say that, “The verb ‘to be’ has the forms am, is and are in the present tense but uses was and were in the past tense.”
There is much more information online about the use of the infinitive as a noun, an adjective and an adverb, However, the information here is sufficient to fulfil the (non statutory) requirement for knowledge of technical grammatical terms used in the Glossary for the programmes of study for English.