這個文法不太懂

這個是past continous tense. 通常会有另一個過去動詞跟尾
e.g. Two weeks ago, while she was eating an apply, the dog ran away.
意思是在Ｚ星期前,她正在吃apple之時, dog仔跑了, 吃蘋果過程較長, 狗仔跑的動作短, 是中途插進去的, 這個 tense 也可用以下形式表達:
e.g. While I was singing a song, my mother was cooking. 這則表達２個過去動作同時進行.
希望你滿意.
英文唔同中文, 能從verb 的形式看出時空,總數有１２個 tense.

It is past continous tense.
For examples,
Mary was eating an apple at 8 last night.
Her sister was doing her homework at 10 last night.
It is mainly use for a certain time in the past ,it is quite easy.
It is like the present continous tense,but this tense is used for the past.

past continuous:is mean at this time of the past.