Summarize:

The present tense is used for anything that's currently happening. To conjugate an -er verb in the present tense, remove the -er ending from the infinitive and add -o, -es, -e, -emos, -eis, or -en, depending on the subject pronoun. Yo como: I eat. Tu comes: You [informal] eat. El/ella/usted come: He/she eats; you [formal] eat. Nosotros comemos: We eat. Vosotros comeis: You all [informal] eat. Ellos/ellas/ustedes comen: They/you all [formal] eat. If you want to talk about something that happened completely in the past, remove the -er ending from the infinitive of your verb and add the appropriate ending. Depending on your subject pronoun, your ending will be -i, -iste, -io, -imos, -isteis, or -ieron. Yo comi: I ate. Tu comiste: You [informal] ate. El/ella/usted comio: He/she/you [formal] ate. Nosotros comimos: We ate. Vosotros comisteis: You [informal] ate. Ellos/ellas/ustedes comieron: They/you [formal] ate. If an action occurred in the past but continues into the present, the imperfect tense is more accurate than the preterite tense. This tense is also appropriate if you "were doing" something, but were interrupted. To conjugate -er verbs in this tense, replace the -er ending with -ia, ias, ia, -iamos, iais, or -ian. Yo comia: I was eating. Tu comias: You [informal] were eating. El/ella/usted comia: He/she was eating; you [formal] were eating. Nosotros comiamos: We were eating. Vosotros comiais: You all [informal] were eating. Ellos/ellas/ustedes comian: They/you all [formal] were eating. If you want to talk about a hypothetical situation, or something that will only occur if something else takes place first, use the conditional tense. To conjugate -er verbs, you'll use the same endings that you used for the imperfect tense. However, you'll attach them to the end of the infinitive, rather than removing the -er ending. Yo comeria: I would eat [if]. Tu comerias: You [informal] would eat [if]. El/ella/usted comeria: He/she/you [formal] would eat [if]. Nosotros comeriamos: We would eat [if]. Vosotros comeriais: You all [informal] would eat [if]. Ellos/ellas/ustedes comerian: They/you all [formal] would eat [if]. Like the conditional tense, verbs are conjugated in simple future tense by adding an ending directly onto the end of the infinitive. Depending on the subject pronoun, you'll add -e, -as, -a, -emos, -eis, or -an. Yo comere: I will eat. Tu comeras: You [informal] will eat. El/ella/usted comera: He/she/you [formal] will eat. Nosotros comerermos: We will eat. Vosotros comereis: You all [informal] will eat. Ellos/ellas/ustedes comeran: They/you all [formal] will eat.
Talk about actions happening in the present. Use the preterite tense for completed past actions. Try the imperfect tense for habitual actions. Move to the conditional tense if the action depends on something else. Form the future tense by adding the appropriate ending.