"Am," "is" and "are," are present-tense forms of the verb "to be," while "was" and "were" are past-tense forms. The perfect form is "(have) been," and the progressive form is "(am) being."
"To be" is one of the most frequently used verbs in the English language, and it is also the most irregular, using a different form often even within a particular tense. The present tense has three different forms, instead of the usual base verb or base verb plus -s of a regular verb. The past tense has two forms instead of the one -ed form of a regular verb.