Giúp 21-25 .

`21`. I broke my pen while I was doing my homework.

`22`. It started to rain when we were walking home from school.

`23`. I realized I didn't have any money when I went to get the tickets.

`24`. My mother was classifying old clothes while my father was preparing the food packages for the next voluntary trip.

`25`. Some volunteers were teaching disabled children while others were repairing some furniture in the orphanage.

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`***` `1` việc đang xảy ra ở quá khứ thì bị hành động khác chen vào:

   QKĐ/QKTD when QKĐ/QKTD

   QKĐ/QKTD while QKĐ/QKTD    

`***` `2` việc xảy ra liên tiếp trong quá khứ:

   QKĐ when QKĐ

`***` `2` việc xảy ra cùng lúc trong quá khứ:

   QKTD while QKTD

`->` QKĐ: S + V-ed/c2

`->` QKTD: S + was/were + V-ing