〜間に

留守(るす)の間(ま)に宅配(たくはい)便(びん)が来(き)ました。

Rusu no aida ni takuhaibin ga kimashita.

A delivery came while I was out.

間に puts the focus on the event inside that stretch

子供(こども)が寝(ね)ている間(ま)に家事(かじ)をします。

Kodomo ga nete iru aida ni kaji o shimasu.

I do the housework while the children sleep.

It often swaps with うちに, but carries no sense of a closing window

この二(ふた)つの違(ちが)いは、助詞(じょし)が一(ひと)つあるかどうかです。

Kono futatsu no chigai wa, joshi ga hitotsu aru ka dō ka desu.

The difference between the two is just whether one particle is there.

間に has the に and marks one event inside the stretch; 間 without it runs throughout