The verb 'to be' is one of the most fundamental verbs in English. In the present tense, it has three forms: 'am', 'is', and 'are'. 'Am' is used with the pronoun 'I' (e.g., I am happy). 'Is' is used with singular nouns and pronouns like 'he', 'she', and 'it' (e.g., He is tall). 'Are' is used with plural nouns and pronouns like 'we', 'you', and 'they' (e.g., They are friends). Understanding these forms is crucial for constructing grammatically correct sentences.