now是表示现在这个时刻,用现在进行时,所以是are having a class meeting
yesterday表示昨天,是一整天的时间,用一般过去时,所以用had a class meeting
而如果是表示昨天的某个时刻,则可以用过去进行时(were having),例如昨天9点钟学生们在开班会:The students were having a class meeting at 9:00 yesterday.
now是表示现在这个时刻,用现在进行时,所以是are having a class meeting
yesterday表示昨天,是一整天的时间,用一般过去时,所以用had a class meeting
而如果是表示昨天的某个时刻,则可以用过去进行时(were having),例如昨天9点钟学生们在开班会:The students were having a class meeting at 9:00 yesterday.