他在读书时常常打瞌睡。

He often doze off while reading.

他在读书时用手支撑着头。

He is propping his head with his hand while reading.

他在读书时在椅子上来回摇动。

He was rocking back and forth in his chair while reading a book.

他在读书时想象着书中的场景。

He was picturing the scene in his mind as he read the book.

昨天的这个时刻他在读书。

He was reading at this moment yesterday.

他在睡觉没在读书。

He was sleeping rather than reading.