Modal verbs indicate possibility, obligation, will, knowledge or intention, and almost always accompany another verb in the infinitive. In Portuguese, they organize the speaker's attitude toward the action, rather than naming the main action. This function makes the modals central to the [Imperative Mood], the [Subjunctive Mood], and the [Future of the Preterite].

Poder expresses possibility, permission and ability, and is one of the most frequent modals in constructions with the infinitive. Its conjugation shows clear irregularity in the first person singular, with forms such as posso, while the general pattern approaches -er verbs. Its use naturally combines with [Auxiliary Verbs] and with [Verbal Periphrases], because it introduces the main verb without changing its lexical value.

PessoaPerson.	FormaForm.	ExemploExample.
1.ª singularFirst person singular.	possoI can.	Posso entrar, se a porta estiver abertaI may enter if the door is open.
2.ª singularSecond person singular.	podesyou can.	Podes sair, se terminares o trabalhoYou may leave if you finish the work.
3.ª singularThird person singular.	podecan.	Pode começar, se quiseres agoraYou may start now if you want.

Dever expresses obligation, moral duty, and strong expectation about a future action. The verb functions as a modal when it appears followed by an infinitive and tends to sound more formal than 'ter que'. Its presence brings the utterance closer to a requirement, which ties it closely to the [Subjunctive Mood] and to more normative registers.

PessoaPerson.	FormaForm.	ExemploExample.
1.ª singularFirst person singular.	devoI must.	Devo estudar, se quiser bons resultadosI must study if I want good results.
2.ª singularSecond person singular.	devesyou should.	Deves chegar cedo, se o encontro for importanteYou should arrive early, if the meeting is important.
3.ª singularThird person singular.	devehe/she must.	Deve respeitar as regras, se entrar na salaYou must respect the rules if you enter the room.

Want expresses desire, intention and will directed toward a concrete action, usually followed by an infinitive. As a modal, it links the speaker's impulse to the main verb without losing the idea of choice or intention. In courteous contexts, it can soften requests and approaches, especially when it appears in [Future of the Preterite].

PessoaPerson.	FormaForm.	ExemploExample.
1.ª singularFirst person singular.	queroI want.	Quero aprender, se tiver tempoI want to learn if I have time.
2.ª singularSecond person singular.	queresyou want.	Queres sair, se a chuva pararDo you want to go out if the rain stops?
3.ª singularThird person singular.	querWant.	Quer viajar, se encontrar companhiaWant to travel if you find company.

Know can function as a modal when it means having the ability to do something or knowing the correct way to do it. It also appears before the infinitive to mark practical competence, and not merely abstract knowledge. Its irregularity in sei brings it close to other very frequent verbs, such as [Irregular Verbs].

PessoaPerson.	FormaForm.	ExemploExample.
1.ª singularFirst person singular.	seiI know.	Sei nadar, se o mar estiver calmoI know how to swim if the sea is calm.
2.ª singularSecond person singular.	sabesyou know.	Sabes responder, se leres com atençãoDo you know how to answer if you read carefully?
3.ª singularThird person singular.	sabeHe/she knows.	Sabe cozinhar, se tiver ingredientesDo you know how to cook if you have ingredients?

Ter que, ter de, and haver de introduce obligation or necessity, but vary in register and regional preference. Ter que is very frequent in everyday language, ter de is often preferred in formal registers or in Portugal, and haver de can add a sense of future, promise, or destiny. These constructions are close to [Auxiliary Verbs] and to [Verbal Periphrases], because they join a supporting verb to a main action in the infinitive.

RegiãoRegion.	Palavra ou ExpressãoWord or Expression.	Definição RegionalRegional Definition.	ExemploExample.
PortugalPortugal.	ter deto have to.	Expressa obrigação com tom frequente em registo formal e cuidado.Expresses obligation with a frequent tone in a formal and careful register.	Tenho de sair, se o autocarro partir cedoI have to leave if the bus leaves early.
PortugalPortugal.	haver deto have to.	Exprime obrigação futura, promessa ou ideia de destino.Expresses a future obligation, a promise, or a sense of destiny.	Hei de voltar, se a ocasião for boaI shall return if the occasion is good.
BrasilBrazil.	ter quehave to.	Expressa obrigação na fala corrente com grande frequência.Expresses obligation in everyday speech very frequently.	Tenho que estudar, se quiser passarI have to study if I want to pass.

Portuguese verbs are organized mainly into three regular conjugations, in -ar, -er and -ir, and the modals often approximate the -er or -ir pattern with internal changes. Regular conjugation helps to recognize endings, but important modals require attention to stem changes and the most frequent irregular forms. This base is useful for understanding [Regular Verbs] and for interpreting the forms that appear in verbal chains.

ConjugaçãoConjugation.	MarcaBrand.	ExemploExample.
PrimeiraFirst.	ar-ar (infinitive ending).	falar, estudar, trabalharto speak, to study, to work.
SegundaSecond.	er-er (infinitive ending).	beber, correr, aprenderto drink, to run, to learn.
TerceiraThird.	ir-ir (infinitive ending).	partir, dormir, subirto leave, to sleep, to go up.

Modals and the verbs that accompany them frequently use non-finite forms to connect ideas and build periphrases. The impersonal infinitive presents only the base verb form, while the personal infinitive agrees with the subject in contexts of greater clarity or formality. The gerund marks continuity, the participle marks result, and these forms are essential for [Verbal Participles].

FormaForm.	TerminaçãoEnding.	ExemploExample.
Infinitivo impessoalImpersonal infinitive.	basebase.	fazer, saber, poderto do, to know, to be able.
Infinitivo pessoalPersonal infinitive.	flexão pessoalPersonal inflection.	para eu fazer, para nós fazermosfor me to do, for us to do.
GerúndioGerund.	ando endo indopresent participle endings: -ando, -endo, -indo.	falando, tendo, partindospeaking, having, leaving.
ParticípioParticiple.	ado ido e irregularesPast participles: -ado, -ido, and irregular.	falado, feito, escritospoken, done, written.

Among the most frequent modal and auxiliary verbs, there are highly irregular forms that appear early in reading and speech. Poder shows posso, saber shows sei, ter shows tenho, haver shows hei, and ir shows vou, and these alternations should be recognized as their own paradigms. Contact with these forms is decisive for understanding [Verbal Moods], because they appear in high-frequency periphrastic constructions.

VerboVerb.	FormaForm.	ExemploExample.
poderCan / Be able.	possoI can.	Posso sair, se terminar agoraI can leave if it ends now.
saberKnow.	seiI know.	Sei fazer, se praticar todos os diasI know how to do it if I practice every day.
terto have.	tenhoI have.	Tenho que ir, se a reunião começarI have to go if the meeting starts.
haverthere is/are.	heiI will have.	Hei de voltar, se for precisoI shall return if it is necessary.
ir-ir (infinitive ending).	vouI will.	Vou começar, se o sinal estiver verdeI will start if the signal is green.

Verbal periphrases link an auxiliary with an infinitive, gerund or participle to create values of obligation, intention, duration, or near future. In European Portuguese, estar a + infinitive is very common, while in Brazilian Portuguese the gerund often occupies this function. These constructions articulate the meaning of the modals with the structure studied in [Auxiliary Verbs] and [Verbal Periphrases].

EstruturaStructure.	ValorValue.	ExemploExample.
poder mais infinitivopoder + infinitive.	possibilidadePossibility.	Pode chover, se as nuvens cresceremIt may rain if the clouds grow.
querer mais infinitivowant + infinitive.	intençãoIntention.	Quero viajar, se houver tempoI want to travel if there is time.
dever mais infinitivodever + infinitive.	obrigaçãoObligation.	Deves responder, se recebeste a mensagemYou should respond if you have received the message.
ir mais infinitivogoing to + infinitive.	futuro próximoNear future.	Vou sair, se o trabalho terminarI will leave if the work ends.
estar a mais infinitivoestar a + infinitive (present progressive)	progressão atualCurrent progression.	Estou a ler, se não houver ruídoI am reading, if there is no noise.
poderia mais infinitivocould + infinitive	cortesiaCourtesy.	Poderia ajudar, se fosse possívelCould you help if it were possible?

Portuguese modals serve to situate the action in terms of possibility, obligation, desire, ability, and intention. Their form depends on regular paradigms, frequent irregularities, and constructions with the infinitive, gerund, and participle. The natural use of these verbs prepares the reading and production of central structures in Imperative Mood, Subjunctive Mood and Future of the Preterite.