Modal verbs are verbs that express the speaker's attitude toward the action, such as possibility, ability, permission, obligation, or deduction. In Portuguese, modals appear as main verbs with complements or verbal periphrases. They do not carry the main sense of the action, but modify the value of the statement. The same modal can have different values depending on the context.

We express possibility in Portuguese with 'poder', 'dever', and 'deve' in certain contexts. We also use 'pode ser que' and 'é possível que', normally with the subjunctive. The choice among these forms indicates how probable we consider the event. Intonation and context help distinguish possibility from deduction.

We express ability with "poder" and "conseguir". "Poder" indicates having means, permission, or conditions to do something. "Conseguir" focuses on the result or the ability to accomplish the task. In many contexts, "saber" also indicates learned ability, especially with verbs like "falar" or "dirigir".

Permission is usually expressed with "poder" in the present or imperfect past to be more courteous. "Permitir" appears as a main verb and usually requires a complement with "que" and the subjunctive or an infinitive. The choice between "poder" and "permitir" depends on whether we are talking about general authorization or an act of authorization.

Obligation appears with "ter que", "precisar" and "dever". "Ter que" is the most neutral and common form in speech. "Precisar" can sound softer and can express practical necessity. "Dever" carries a tone of moral obligation, strong recommendation, or high probability, depending on the context.

Deduction usually uses "dever" in the present or the perfect tense, especially in writing and in more formal records. Constructions like "deve ser" also appear for the speaker's inferences. The meaning is a logical conclusion with high probability, not obligation. Context differentiates deduction from obligation.

Portuguese uses periphrases such as "é preciso" and "tem que" to express modality. In these constructions, a fixed verb or expression functions as a modal and the other verb appears in the infinitive. "É provável que" and "é possível que" usually use the subjunctive in the following verb. These periphrases organize the sentence to highlight necessity or possibility.

Many expressions of possibility and necessity pair with the subjunctive because they deal with facts not guaranteed. After "it's possible that", "it's likely that", and "it may be that", the subordinate verb tends to appear in the subjunctive. With "ter que" and "precisar", we normally use the infinitive, not the subjunctive. The choice between subjunctive and infinitive depends on the structure chosen to modalize the sentence.

Modal usage can vary in frequency and form according to register and region. 'Ter que' is very frequent in informal speech. 'Dever' is more common in writing and in formal contexts for obligation or deduction. 'Precisar de' with an infinitive appears in some varieties, but 'precisar' without 'de' is the standard form with the infinitive.