An irregular verb in Portuguese is a verb whose stem or endings change in unpredictable ways in certain tenses or persons. These changes do not follow the regular pattern of -ar, -er or -ir endings. Therefore, the exact form must be memorized with examples. The verbs ser, estar, ter and ir are the main ones for everyday communication.

In the present, common irregular verbs show changes in the first person singular and in some singular persons. These forms express habitual actions, states and general truths. Memorizing the common forms allows you to talk about everyday life with clarity.

In the simple past, many irregular verbs change the stem in I and, sometimes, in he or we. This tense narrates completed actions in the past. The forms fui, estive, tive and veio are essential for telling what has already happened.

In the future of the present, many verbs keep the irregular stem but follow regular future endings. This tense expresses predictions, plans and promises. The forms serei, estarei, terei and irei appear frequently in commitments and intentions.

The verb to be is highly irregular and expresses identity, profession, origin, characteristics and times. In the present and the simple past, the forms change in unpredictable ways. In the future, the stem is be- with regular future endings.

The verb estar indicates a temporary state, location and momentary conditions. In the present, irregularity appears in estoy and está. In the simple past, the key form is estive for I and esteve for he or she.

The verb to have expresses possession, obligation and experience. In the present, the important difference is between tem and têm. In the simple past, tive is the form for I, and teve is the form for he or she.

The verb to go indicates movement, intention and near future. In the present, vaya is irregular in the first person singular. In the past, fui is used for I, and fue for he or she.

The verb to come is irregular because of the forms vim in the simple past and came in the third-person singular. In the present, the form I come also deviates from the regular pattern. In the future, the stem is 'vir-' with future endings.

The verb fazer is irregular as I do (faço) in the present for I and fez in the simple past for he or she. This verb appears a lot when talking about activities, time and weather. In the future, the stem is far- with regular future endings.

The verb dizer is irregular in digo in the present for I and said in the simple past. It is used to report speech, opinions and information. In the future of the present, the stem is dir- with future endings.

Many irregular verbs share similarities in the groups of endings, even with stem changes. -zer and -ger tend to change to a dental sound in order to keep the sound, as in I do and I say. Practicing with full sentences helps fix the correct forms in real use.

There is regional variation in the use of tú, você and the associated conjugation, especially in the present. In Brazil, você typically uses the third-person singular conjugation, even though it is informal. In Portugal, tú with és, tens and fazes is more common in informal conversation.