What’s the difference? した vs していた

Since していました is used to indicate that an action took place during a specific time, it is often accompanied by a time or else another action or event that occurred at a certain time.

昨日、勉強しました。              昨日の２時ごろ、勉強していました。

Kinou, benkyō shimashita.       Kinou no ni-ji goro, benkyō shite imashita.

I studied yesterday.                  Around two o’clock yesterday, I was studying.

Also note that ～ている cannot be used with momentary actions because it implies a continuous action.

授業中に、寝ていました。          授業中に、消しゴムを落としました。

Jyugyō-chuu ni, nete imashita.    Jyugyō-chuu ni keshigomu o otoshimashita.

I was sleeping during class.         I dropped my eraser during class.