When the teacher came into the classroom,he 【prepared/planned】to be doing his homework.
当老师进入教室的时候,他准备/打算要做作业.
前后的句子时态要保持一直 都用一般过去时.prepare/plan to do sth 准备/计划做某事
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书山有路勤为径,学海无涯苦作舟.
-----------与君共勉-----------