in one clause during an action or state expressed in another clause. For example: (They were listening to the radio when they heard a noise outside.) (I was in my room when you came in.) (It was a very peaceful day when that happened.) In all three cases, an event or completed action interrupts an ongoing state or action. For example, in the second sentence, the speaker states that he was in his room (expressed through the imperfect to reflect the ongoing or unfinished state of being there) when the other person "interrupted" that state by entering (expressed through the preterite to suggest a completed action). Present progressive and imperfect progressive The present and imperfect progressive both are used to express ongoing, progressive action in the present and past, respectively. For example: (I am doing my homework) (We are studying) (I was listening to the radio) (He was cleaning his room) The present progressive is formed by first conjugating the verb or , depending on context, to agree with the subject, and then attaching a gerund of the verb that follows. The past (imperfect) progressive simply requires the or to be conjugated, depending on context, in imperfect, with respect to the subject. Forming gerunds To form the gerund of an verb, replace the of the infinitive with ; e.g. , , → , , . For or verbs, replace the or ending with ; e.g. , , → , , (note that undergoes the stem vowel change that is typical of verbs). In verbs whose stem ends with a vowel, the of the ending is replaced by : e.g. , , → , , . In verbs whose stem ends with —such as and —the stem vowel is raised to (as is typical of verbs), and this merges with the of the ending; e.g. , → , . Subjunctive The subjunctive of a verb is used to express certain connotations in sentences such as a wish or desire, a demand, an emotion, uncertainty, or doubt. Present subjunctive Normally, a verb would be conjugated in the present indicative to indicate an event in the present frame of time. (I am very ambitious) (Marta brings the food) If the sentence expresses a desire, demand, or emotion, or something similar, in the present tense, the subjunctive is used. (I want you to be very ambitious—literally, I want that you be very ambitious) (I