んです — when should I actually add this vs just using plain です/ます?

Question

I get that んです adds 'explanation nuance' but in practice I never know when a sentence NEEDS it. Are there clear trigger situations, like answering a 'why' question?

Accepted Answer

Yes, the two biggest trigger situations: (1) answering/asking 'why' — どうして遅れたんですか / 電車が止まったんです ('the reason is the train stopped'), and (2) pointing out something you just noticed and want to comment on or ask about — あれ、財布がないんです ('huh, [I've noticed] my wallet isn't here'). Plain です/ます just states a fact neutrally; んです signals 'there's a story/reason behind this that I want to share or that you should know'. If you're just giving a flat fact with no explanatory context (今日は月曜日です), you don't need it.