They teach you how to teach grammar but not grammar itself. It's the same for all teaching certifications, I should think :)

Hi, A native English speaker generally knows grammar, so the TEFL certification is about how to teach the grammar points. Non-native English teachers should also have an IELTS, or similar certification, showing that they have a verified C1+ level of English. If they do, they will also know the grammar points. There are good TEFL certificates and CELTA qualifications, but there are also cheap $20 TEFL certificates that don't teach much at all.

I have a CELTA certificate, which is one of the more expensive and thorough certificates for TESL one can get. This course, like most TESL certifications, does not teach grammar.

Hello! I'm an English graduate and of course all forms of language were taught in my degree. We learned about grammar, phonetics, anglophone history and civilisation, culture, sociolinguistics, linguistics and literature! In my country, many people do an English degree in order to become English teachers, and any English teacher has to know a lot about grammar in order to explain grammar to students and help them have a better understanding of the language! I hope my response was helpful!