Conjugation (conjugación) involves changing the ending of verbs to express tense, mood, aspect, as well as person. Spanish mainly drops the subject pronoun and the subject is reflected in the verb conjugation.

Spanish verbs are commonly divided into three groups based on their infinitive endings which determines how they are conjugated in each tense:

Apart from the infinitive ending, which is often dropped in conjugation, each verb also has a stem, which is the part that conveys the sense of the verb. While the infinitive ending determines how the verb is conjugated, the stem is the part that actually undergoes conjugation by taking the specific endings for each person and tense. To obtain the verb stem, simply remove the infinitive marker -ar, -er, or -ir. Sometimes, verb stems undergo changes in conjugation. The examples below show how different verbs undergo conjugation for third-person singular in the indicative present tense:

Simple tenses are those that simply conjugate the main verb, meaning the verb has the form of a single word in these tenses. Compound tenses, on the other hand, include a conjugated auxiliary verb and a present or past participle form, which means the verb is not a single word in these tenses. In Spanish, the progressive and perfect tenses have compound forms. In these tenses, the auxiliary ("estar" or "haber") is conjugated for person and tense while the main verb is in a participle form. For example:

The Spanish tenses differ with each other in the number of different forms a verb can take when conjugated. For example, the first-person and third-person singular share the same form in some tenses (the imperfect, conditional simple, present subjunctive, and imperfect subjunctive). There is also an informal plural second-person (vosotros) conjugation which is only used in Spain but not in Latin America. The tenses also differ in the number of verbs that have irregular conjugation.

The present tense has 6 different forms for each verb (counting the "vosotros" form) and has three different possible sets of endings based on the form of the infinitive. The table below shows the conjugation of a sample verb from each type of infinitive (hablar, comer, vivir):

Some verbs have irregular conjugation in the present tense. While some of them are completely irregular (like "ir" and "ser"), others are only irregular in the first-person singular (like "conocer"), and still others undergo stem changes while taking the regular endings (like "pedir").

Each verb can have 6 different forms in the present simple (counting the "vosotros" form) and there are two possible sets of endings based on the form of the infinitive:

Note that the first-person plural (nosotros/as) form of past simple verbs are similar to their present tense counterparts for -ar and -ir forms.

The past simple has the largest number of irregular verbs out of all the tenses in Spanish. These include verbs that are completely irregular (like "ser" and "ir"), verbs that have irregular endings (like "estar"), and verbs that undergo stem change (like "tener" and "saber").

The imperfect conjugation has the same form in the first-person and third-person singular, which means there are 5 different forms for each verb (counting the "vosotros" form). The verbs endings are different for -ar verbs and -er/-ir verbs:

The imperfect is one of the least irregular tenses in Spanish and only three verbs have irregular conjugation in this tense: "ser," "ir," and "ver."

Each verb has 6 different forms in future simple conjugation. The conjugation in this tense is different from other tenses because the verbs endings are added to the infinitive rather than the stem and are the same for all three classes of infinitives.

The future simple has no completely irregular verbs, but some verbs undergo stem changes (like "tener," "hacer," "haber," etc.).

Verbs have the same form in the first-person and third-person singular of conditional simple tense which means each verb has 5 different forms (counting the "vosotros" form). There is a single set of endings that applies to all classes of verbs, and like the future simple, these endings are added to the infinitive rather than the verb stem.

The conditional simple shares the stem change verbs with the future simple and has no other irregular forms.

The imperative borrows all its verb forms from other tenses, namely the present indicative and present subjunctive. So there are technically no new forms here, except for the affirmative "vosotros" form which replaces the "-r" at the end of the infinitive form with "-d." The imperative is only used with second-person subjects, but has separate affirmative and negative forms, although the affirmative and negative forms are the same for "usted" and "ustedes." This means that, overall, verbs can have 6 different forms in the imperative based on number and level of formality.

The imperative has 8 irregular verbs in the affirmative "tú" form (decir, hacer, ir, poner, salir, ser, tener, venir). Other irregularities are similar to the present indicative and present subjunctive.

Each verb can have 5 different forms in the present subjunctive because the first- and third-person singular forms are similar. There are two different sets of endings for -ar and -er/-ir verbs which involve use of the opposite vowel.

Any verb that has irregularities in the first-person singular of present indicative (like "tener" and "conocer") will carry over the irregularity to the present subjunctive. There are also special irregular forms for verbs like "ir" and "ser" and verbs that undergo stems changes (like "pensar" and "dormir").

There are 5 possible forms for each verb in the imperfect subjunctive because the first- and third-person singular forms are the same. Imperfect subjunctive has a single set of endings for all verb classes.

There are no special irregular forms in the imperfect subjunctive and all irregularities are carried over form the past simple tense whose third-person singular form is used to obtain the imperfect subjunctive stem.

Participles do not have separate forms for different subjects when used as part of compound tenses. However, they take different endings based on the infinitive types and there is a present and a past form for each verb: