The verb "be" is the second most common word in English and serves both as a main verb meaning "to exist" and as an auxiliary that forms progressive tenses and passive voice. Its forms are "is" (third-person singular present), "was" (first/third-person singular past), "were" (second person singular and all plurals in the past), and "been" (the past participle used in perfect tenses). The past participle "been" combines with forms of "have" to make perfect tenses: "I have been waiting," "she had been working."
The verb "have" serves as a main verb meaning "to possess" and as an auxiliary forming perfect tenses. Its forms include "has" (third-person singular present) and "had" (past tense and past participle). "Have you finished?" uses "have" as an auxiliary. The verb "do" functions primarily as an auxiliary for questions ("Do you like coffee?") and negation ("I do not know"), though it can also be a main verb meaning "to perform" ("Do your homework"). The adverb "not" negates these and other verbs, often forming contractions like "isn't," "wasn't," "doesn't," "can't," "won't," and "shouldn't."
Modal verbs express possibility, permission, obligation, or future intention. "Will" indicates future tense or strong volition, as in "I will go tomorrow." "Would" can refer to a conditional outcome ("I would help if I could"), a polite request ("Would you pass the salt?"), or a habitual past action ("She would read every night"). "Can" expresses ability or permission ("I can swim"); its past form "could" expresses past ability or polite possibility. "Must" conveys obligation or strong probability ("You must try this," "He must be tired"). "Should" gives advice, expectation, or mild obligation ("You should rest," "The train should arrive soon"). Modals do not change form for third-person singular ("he can," not "he cans") and are followed by the base form of the verb.