Modal verbs express the speaker's attitude toward what they say. In Portuguese, they appear as main verbs with a modal sense or as periphrastic constructions with "ter que", "poder", "dever" and others. Modals indicate possibility, obligation, permission, necessity, or intention. The modal value depends on the context and the choice of the verb.

To indicate possibility, we use 'poder' as the main verb, periphrastic constructions with "é possível" and also constructions with "dever" in interpretive reading. The context decides whether the possibility is stronger or weaker. The sentence can combine modals with adverbs to adjust the degree of certainty.

Permission typically uses 'poder' in the present or imperfect past for greater courtesy. 'Deixar' also expresses permission, usually with a person as the object. The tone of the sentence can change with vocatives and with the use of questions. In formal registers, alternatives such as "it is permitted" appear.

Ability uses "poder" for circumstantial ability and "saber" for learned ability. "Conseguir" highlights success in performing an action, often in specific situations. The context can alternate between possibility and ability without changing the verb. The choice between "poder" and "saber" changes the focus of the information.

Obligation typically appears with "ter que" as a periphrasis, with "precisar" and with "dever" in more formal registers or with a sense of recommendation. "Ter que" is the most direct form in everyday speech. "Dever" may sound like advice or expectation, depending on the context. Negation changes the meaning to the absence of obligation.

Necessity uses 'precisar' as the main verb and also appears in constructions with 'é necessário'. The sentence may omit the impersonal subject to generalize. Necessity approaches obligation when it involves commitments or deadlines. The complement of the verb indicates what needs to be done.

Intention usually uses "querer" for will and "ir" + infinitive for near-future plan. "Pretender" appears as a planned intention, with a more formal tone. The choice of verb tense can indicate proximity or certainty of the plan. Intention can coexist with possibility in the same sentence.

The main modals appear as regular or semi-irregular verbs in the present tense: 'poder', 'dever', 'precisar', 'querer', and the periphrases with 'ter que'. 'Poder' and 'querer' have irregularities in some present-tense forms. Periphrases keep the first verb conjugated and the second in the infinitive. Agreement follows the subject of the sentence.

The negation of modals is formed with 'not' before the main verb or the periphrasis. In 'ter que', the typical negation is 'not have to', which means there is no obligation. In 'poder', the negation results in prohibition or impossibility, depending on context. The focus of the negation can change with the order of elements in the sentence.

The central modals in Portuguese are 'poder' (possibility and permission), 'ter que' and 'precisar' (obligation and necessity), 'dever' (obligation or advice) and 'querer' (intention). The exact meaning depends on context and the register of speech. Periphrases such as 'ir + infinitive' express near-future planned action without needing a separate modal verb. The appropriate choice makes communication clearer and more natural.