I understand them too, because anyone who grasps a good grammar in his/her native language understands easily the theory. To apply the knowledge is harder. My daughter talks a reasonably good English, of course more polite, less slang, at home otherwise I start to use any strange word like "***- " till I learn from her reaction what exactly it's the meaning, if positive or negative, pejorative or not and the intensity of the word because there I have a lot of problems and create misunderstandings. Even if I make a lot of mistakes and I could not express myself well, I can still see her mistakes (same for sites; even for Princeton dictionary with the mis-spelt word "syntax" - of course a typo) and the vocabulary/or understanding the meaning of words that are less used, is greater than her. Of course because of my language the majority of people look down on me.
One day my sister told me that is easy to write well in anyone mother's language and I said "Do you want me to correct your emails? You wouldn't like it, because it is not Swedish, is your mother language and we have the same mother."(I meant education!)
In my homeland I have had friends that were doctors, dentists, even teachers who made mistakes and I fussed about like an idiot any time they brag about their titles ... while they were great specialists in their fields and that was really important!!

It's worse and you talk perfectly but you can't communicate because some people do not get the message no matter they have a good grammar and a large, wide vocabulary.

It took me to immigrate here to change my point of view. :-)