What grammar topic has genuinely stumped you, at any level?
I teach Spanish as a foreign language and I've been thinking about this a lot lately. We as teachers tend to assume we know which topics are the hardest (subjunctive, ser vs. estar, the preterite/imperfect distinction) but I wonder how much that matches what learners actually experience.
So I wanted to ask directly: what's the grammar point that has cost you the most, whether it's something you struggled with early on or something that still trips you up now? Any level, any background.
It doesn't have to be a "big" topic either. Sometimes it's something small and specific (a particular verb, a pronoun rule, a preposition) that just never quite clicked.
Every time I ask this I end up surprised, there's never one obvious answer.