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, -éis, or -en, depending on the subject pronoun.   Yo como: I eat.  Tú comes: You [informal] eat.  Él/ella/usted come: He/she eats; you [formal] eat.  Nosotros comemos: We eat.  Vosotros coméis: 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 -í, -iste, -ió, -imos, -isteis, or -ieron.   Yo comí: I ate.  Tú comiste: You [informal] ate.  Él/ella/usted comió: 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 -ía, ías, ía, -íamos, íais, or -ían.   Yo comía: I was eating.  Tú comías: You [informal] were eating.  Él/ella/usted comía: He/she was eating; you [formal] were eating.  Nosotros comíamos: We were eating.  Vosotros comíais: You all [informal] were eating.  Ellos/ellas/ustedes comían: 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 comería: I would eat [if].  Tú comerías: You [informal] would eat [if].  Él/ella/usted comería: He/she/you [formal] would eat [if].  Nosotros comeríamos: We would eat [if].  Vosotros comeríais: You all [informal] would eat [if].  Ellos/ellas/ustedes comerían: 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 -é, -ás, -á, -emos, -éis, or -án.   Yo comeré: I will eat.  Tú comerás: You [informal] will eat.  Él/ella/usted comerá: He/she/you [formal] will eat.  Nosotros comerermos: We will eat.  Vosotros comeréis: You all [informal] will eat.  Ellos/ellas/ustedes comerán: 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.