at that time指某一個回憶圖像的時間點(diǎn),這個點(diǎn)在過去的某一刻,此處應(yīng)翻譯為“當(dāng)時”,根據(jù)圖像,他當(dāng)時“正在”忙,用he was working更具體,如果用he worked in the library,這句話的意思是他原來在圖書館工作,（即現(xiàn)在在哪工作就不一定了）和他“當(dāng)時”是否在工作就沒什么關(guān)系了.
所以有這幾種情況“
他當(dāng)時正在工作he was working in the library
他原來在那工作he worked in the library或he used to work in...
他已經(jīng)在那工作好幾年了he has been working in the library for several years.
他現(xiàn)在仍在這里工作he's still working in the library
他的職業(yè)是...he works in a library
出那件事之前他在那里工作（即現(xiàn)在不在了）he had been working in the library before it happened.
如果他不是因為那件事就會在圖書館工作了（即因為那件事所以沒在圖書館工作成）he would have worked in the library if it wasn't for that thing.