As an aspiring teacher of EFL, it’s extremely necessary for you to know the grammatical structure of English, not only from the point of view of traditional grammar, but also from the modern linguistic approach that goes deeper and explains a lot of things that are not explicitly taught.
Some pundits will claim that teaching grammar is not necessary, claiming that the students don’t show improved grammar “at the end of the study”, but frequently these studies don’t last long enough for the grammar to soak into the subconscious and form part of the student’s interlanguage.
Students need some explicit grammar training, partly because it gives them a target to shoot for, partly because if trains their “monitor” and allows them to correct themselves, and partly because it makes them aware of structures in the language that they may not otherwise have detected. If they know about these structures, they will notice them in their environments, and they will acquire them sooner.
One example of the latter situation is how most of my ESL students never detect the past perfect tense, because in speech the auxiliary verbs are contracted and hard to hear. Someone needs to point out to them that the tense exists. Likewise, they don’t hear the contracted auxiliaries in the future with going to. They think that native speakers are saying, “I going to do that,” “He going to be here,” etc. If they aren’t taught the auxiliaries are there, they might not acquire them.
One reason the teacher needs to know the grammar of English is that it assures that she won’t give stupid answers like, “That’s just the way we say it!”
Of course, it’s not absolutely necessary to know the grammar of the students’ native language. (In my classes, only a genius with a lot of spare time would be able to learn all those languages.) However, knowing the grammar of the student’s language helps you detect problems early and see why students are miscomprehending English grammar.
But since we can’t learn all the grammar of all those languages, a really good EFL instructor should learn phonetics, phonology, syntax and other aspects of language to a high level, along with acquiring a functional command of at least two foreign languages.