Learning a language is all about changing habits in your brain. I think that some techniques that people use like studying long vocabulary lists, memorizing phrasal verbs, even this focus on grammar, is an attempt to short circuit a process which actually takes a long time. It takes a lot of exposure, a lot of reading and listening and, eventually, speaking in order to create new habits.
I have been teaching English in Spain for 7 years and some of the main difficulties that Spanish people have in English are:
1) First of all, the third person singular in the present tense, like “he goes” instead of “he go”. That’s common not only for Spanish speakers but for lots of people. Even though that’s easy to explain, it takes a long time for students to get used to.
2) Spanish students tend to overuse prepositions especially after modal or auxiliary verbs (must to go, would to go) or to misuse it when other prepositions should be used (to home instead of at home). Again, learning it well takes practise and time.
In fact, prepositions may be the most disliked part of the English language for many learners. When faced with a difficult challenge such as prepositions, students have two options. Option one is to become extremely frustrated and decide that English is crazy and give up. Or, they could just accept the fact that learning a language is a process, not a marathon, and learn these little monsters one relaxed step at a time. Prepositions are simply different in English and Spanish, and learning all of the prepositions by using a list is the worst thing you can do. Learn a few prepositions in context every day.
3) Another difficult aspect of learning English is the lack of rules or an excess of exceptions. I know, English would be so much easier if we just stuck to the rules! For example, the confusion with articles, "the" or no "the" is always problematic for Spanish speakers because the use of "the" in Spanish is much more common than in English.
4) A missing subject is another common mistake that happens because the subject in Spanish isn’t always necessary. In English, however, it most definitely is! In English, you must specify the subject. It makes a big difference!
So, in brief, studying with a grammar book can help students with all these problems. But I personally don’t rely on the ability to kind of deduce the grammar as a shortcut to learning English. Even if you understand the explanation, which is not always a given, you still have to put in the time of listening, reading and speaking to create some new habits so that, gradually, the brain starts to create these patterns, enabling you to speak more naturally and fluently.
The same is true for people who say "I still don’t understand". "I don’t understand movies. I don’t understand this series". "I was with a group of people last night and I didn’t feel that I spoke as well as I should", all this kind of stuff. It takes time, the language remains fuzzy. If the speaker is still stuck in old habits, it gets demotivating after a while. it’s only through a lot of exposure that he/she is going to form new habits.
There’s nothing wrong with some of the other aids to learning, like language apps if people enjoy using them, but students shouldn´t think that’s going to short circuit the system and enable them to suddenly speak well or understand well. That´s not the way it works, I think. Perhaps for some people, but that’s certainly not my experience.
So, dear Spanish students: Keep at it and you will eventually improve. Consistency and patience. :)