write + it's that…
kaku n desu
it's that…; you see — marks the utterance as an explanation
遅れてすみません。電車が止まっていたんです。
okurete sumimasen. densha ga tomatte ita n desu.
Sorry I'm late — the trains had stopped, you see. neutral
その荷物、どうしたんですか。
sono nimotsu, dou shita n desu ka.
What's the story with all that luggage? neutral