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Use the present tense to describe actions currently happening. Switch to the preterite tense if the action happened in the past. Move to the imperfect tense if a past action continues to happen. Add the imperfect ending to the infinitive to express the conditional tense. Pick the future tense for talking about actions that will happen.
Remove the -ir ending from the infinitive and replace it with the appropriate ending to talk about present actions. The -ir endings are -o, -es, -e, -imos, ís, and -en. For example:   Yo abro: I open.  Tú abres: You [informal] open.  Él/ella/usted abre: He/she opens; you [formal] open.  Nosotros abrimos: We open.  Vosotros abrís: You all [informal] open.  Ellos/ellas/ustedes abren: They/you all [formal] open. When you're discussing an action that doesn't extend at all into the present, the preterite tense is appropriate. Depending on your subject pronoun, you will replace the -ir with -í, -iste, -ió, -imos, -isteis, or -ieron.   Yo abrí: I opened.  Tú abriste: You [informal] opened.  Él/ella/usted abrió: He/she/you [formal] opened.  Nosotros abrimos: We opened.  Vosotros abristeis: You all [informal] opened.  Ellos/ellas/ustedes abrieron: They/you all [formal] opened. When you're talking about something that happens habitually, or that was interrupted, conjugate your verb in the imperfect tense. For an -ir verb, this means replacing the -ir with -ía, ías, ía, -íamos, íais, or -ían.   Yo abría: I was opening.  Tú abrías: You [informal] were opening.  Él/ella/usted abría: He/she was opening; you [formal] were opening.  Nosotros abríamos: We were opening.  Vosotros abríais: You all [informal] were opening.  Ellos/ellas/ustedes abrían: They/you all [formal] were opening. In Spanish, the conditional tense is used to describe actions that might happen, or that are dependent on some other condition to happen. Regular -ir verbs are conjugated using the same endings as the imperfect tense, added to the end of the infinitive rather than the verb stem.   Yo abriría: I would open [if].  Tú abrirías: You [informal] would open [if].  Él/ella/usted abriría: He/she/you [formal] would open [if].  Nosotros abriríamos: We would open [if].  Vosotros abriríais: You all [informal] would open [if]. Ellos/ellas/ustedes abrirían: They/you all [formal] would open [if]. Add an ending to the full infinitive to conjugate regular -ir verbs in future tense, just as you did with the conditional tense. Use the endings -é, -ás, -á, -emos, -éis, and -án.   Yo abriré: I will open.  Tú abrirás: You [informal] will open.  Él/ella/usted abrirá: He/she/you [formal] will open.  Nosotros abriremos: We will open.  Vosotros abriréis: You all [informal] will open.  Ellos/ellas/ustedes abrirán: They/you all [formal] will open.