В приведенном ниже тексте на место пропуска (8) нужно добавить слово ... The teacher 1) ... (to come) into the classroom unusually early and one of the boys who 2) ... (to smoke) a cigarette, 3)... (to have) no time to put it out. So he 4) ... (to throw) it into the desk. A little later the teacher 5) ... (to notice) the smoke 6) ... (to rise) from the desk. "You 7) ... (to smoke) when 18) ... (to come in)?" he 9) ... (to ask).
The sentence describes a past event. The teacher is asking about something that happened in the past. Therefore, the past tense "came in" is the correct choice for the blank (8).