The Japanese, of course, spend a lot of energy learning English, with most taking six years of the language, or up to ten if they take it at university. While Japanese generally aren't the most effective speakers of English, most of them have a cerebral knowledge of the grammar of the language, quite possibly surpassing many of us. Depending on your background, you probably aren't up on what the difference between infinitive and gerunds is linguistically, and never consciously wonder if you should use a "past perfect continuous" verb when speaking or if you can get away with a "conditional progressive" form instead, but many Japanese people do. They're so analytical when it comes to English grammar that I was banned from helping my son when he was studying for his high school entrance exam, being told my natural, living English had no place on a Japanese style test. I have a Japanese friend who's an English teacher, and he always hits me up with questions like, what are the quantitative differences between "should" and "ought to." I tell him they both sound about the same to my ear, but this drives him crazy.
Japanese have extensive knowledge of English grammar.