A direct object pronoun replaces a direct object, that is a noun that directly receives the action of a verb in a sentence. Moreover, direct object pronouns in Spanish agree in number and gender with the nouns they replace. Table below shows the different direct object pronouns.
A. ¿Ves a Pablo y a Juan?
Yo los veo
Here Juan and Pablo directly receive the action of the verb ver (to see). Since Pablo and Juan matches the third person plural, we choose the direct object pronoun that also matches this person, which is los. Therefore. we have replaced Juan and Pablo in this sentence by los. This sentence is given in the simple present.
B. ¿Compraste un regalo?
Lo compré
Here un regalo directly receive the action of the verb comprar (to buy). Since un regalo matches the third person singular, we choose the direct object pronoun that also matches this person, which is lo. Therefore, we have replaced un regalo in this sentence by lo. This sentence is given in the preterite. The preterite tense is one of two simple past tenses.
C. ¿Me llamaste por teléfono ayer?
Te llamé por teléfono ayer
Here tú (you) directly receive the action of the verb llamar (to call up). So the word llamaste tell us that in fact you receive this action. To give a reply to this question, we conjugate the verb llamar for the second person singular in the preterite tense and add te before this conjugated verb.
