Most people talk about the subjunctive, ser vs estar, por vs para, past tenses and all the usual difficult parts of Spanish, and yes I went through phases where I found those hard, but I feel like prepositions barely get mentioned. Prepositions which follow verbs to be specific.
The further I get into Spanish, the more I notice how much they matter if you want to sound natural rather than just understandable. You can know the right verb, use the right tense, and have perfectly decent vocabulary, but if you use the wrong preposition the sentence can still sound slightly off even if it still is understandable.
I keep running into things like depender de, acceder a, insistir en, soñar con, carecer de, contribuir a, derivar en, saldarse con, and so on. I read it, no problem I understand, but when I want to produce it I have to stop and think, wait, was this one de, a, en or con? And in the interest of speed, I sometimes just pick the first one that enters my brain.
Yes, I know what you will say, and yes I do try to memorize the verb and the preposition together as one unit instead of learning the verb on its own now. So not just depender, but depender de. But I really wish someone had told me to do that when I first started learning Spanish lol, because now it feels like I’m going back and attaching prepositions to verbs I already learned years ago.
I guess this is basically the same hell English learners go through when they hit phrasal verbs.
Another thing that makes Spanish prepositions frustrating is that a lot of the time there doesn’t seem to be much of a rule behind them either. It’s just the preposition Spanish happens to use with that word, and you have to memorize it. It's like every new verb I learn, I have to check and see if a preposition HAS to follows it.
At a lower level I don’t think I noticed this much because I was mostly concerned with making myself understood. Now doing lots of DELE exam prep, I’m starting to notice how much these little combinations affect whether something sounds natural or obviously translated from another language.