こんにちは

There are two distinct ways to make a statement in Japanese. One way is to simply report the facts as they are observed. This is the mode of speech that we have learned so far. In this lesson, we will learn a new way: the mode of explaining things.

A report is an isolated description of a fact. When you are late for an appointment, you can already report in Japanese what has happened, バスが来ませんでした。This sentence, however, does not have the right apologetic tone, because it is not offered as an explanation for anything. If you want to mention the busses failing to run on time as an excuse for being late, you will need to use the explanation mode of speech, and say:

バスが来なかったんです。 (As it happens,) the bus didn't come.

An explanation has two components, one that is explicitly described in the sentence (the bus not coming), and another, which is implied, or explained, by it (you being late for the appointment). The sentence-final expression んです serves as the link between what the sentence says and what it accounts for. Compare: