Most people, especially those who are monolingual, don’t realize what the language they speak actually is. Here is a very broad definition (of my own) of the term language:
For example, if I say “Hey, what’s up?” to an English speaker, he will understand that it is a form of greeting, that the conversation is rather informal, and that I am awaiting a brief response summarizing what happened to him lately. None of these mind processes would take place in the mind of someone who doesn’t understand English.
However, there’s no logical reason why “Hey, what’s up?” should cause that. It would be much more logical if the other person looked up and answered: “The ceiling.” The only reason for such a reaction is that we have learned to react in such a way.
And this is how you should think about every language. You should try to understand what effect on your mind an expression you hear or read should have. It doesn’t matter how illogical its construction may seem from the viewpoint of the languages you already know; if you see the reaction it evokes in the speakers of the language, try to take it as a fact that this is the effect it should have on you too.
Since most textbooks won’t teach you how to get a ‘feel’ for a language, one has to find other means of learning. If you have a bunch of native speakers willing to talk to you and explain things you did not understand, then this is probably the best method for you. Otherwise, a good way is to watch films in the target language with subtitles in the very same language because you can see the reaction of other people to what someone said, and the subtitles will help you to remember the phrase.
How does grammar fit into this concept?
So, what’s the point of learning the grammatical rules of a language? It’s very simple; to sound natural. If I say “he do his homework now”, most people will understand that I mean that “he is doing his homework now”, i.e. it conveys the meaning, but most educated speakers will, in addition to that, think that my English is bad or start to mentally reformulate the sentence, trying to understand why I used “do” instead of “does” or “is doing”. None of these effects is desired if you just want the listener to notice the fact that “he is doing his homework now”.
People are used to receive information in a certain form, and if the form differs from what they are used to, it interferes with the reception. Grammar is therefore just a set of rules telling you what sounds natural to an educated native speaker’s ear. This doesn’t mean that you always have to follow these rules; the point is just what impression you would like to leave. For example, imagine you take part in the following conversation:
You: Breakfast? Ain’t nobody got time for that.
It’s completely irrelevant how “ungrammatical” your reply was. The point is that if your friend knows the popular meme (just Google “ain’t nobody got time for that” if you don’t know what this all is about), he will understand the reference and perhaps find your answer humorous. If this was your intention, your use of English was completely appropriate.
Does this concept apply only to natural languages?
In short, no. For example, music is a language, according to the definition above. To be able to listen to music means to learn its language; for example, modern popular music is very simple in terms of structure and “vocabulary”, but it is still very similar to classical music, which makes it possible for musically uneducated people to enjoy many classical works even if they do not understand all the elements it contains; the feeling it creates in them is approximately the intended one.
On the other hand, Arabic music and western music are completely different, both in structure and in “vocabulary”. I admit that I do not enjoy listening to Arabic music. But I cannot say that it is in any way inferior; it would be like saying that Chinese poetry is bad, even though I know know very little Chinese. Before I learn Chinese to a level comparable to that of a native speaker, I cannot judge the impression Chinese poetry makes.