English verbs may have different forms, depending on the subject of the verb, and depending on when the action expressed by the verb takes place.

In the following sentences, the subjects of the verbs indicate who or what is performing the actions expressed by the verbs. The verbs in these examples are underlined.
e.g. We live in the city.
      He lives on Queen Street.

These examples illustrate how the form of a verb may vary, depending on the subject of the verb. In the first example, the subject is we, and the form of the verb is live. In the second example, the subject is he, and the form of the verb is lives.

The different verb forms which indicate when the action expressed by a verb takes place are usually referred to as tenses.
e.g. We always walk to work.
      We walked to work yesterday.

In the first sentence, the verb walk is in the Simple Present tense. In the second sentence, the verb walked is in the Simple Past tense. Present tenses are usually used to express actions which are taking place in the present; whereas past tenses are usually used to express actions which took place in the past.

The infinitive form of a verb can be used without reference to any particular subject or any particular time. In English, the infinitive form of a verb begins with the word to. For instance, to walk is the infinitive of the verb used in the two preceding examples.