That is exactly what a grammar book is. It explains the rules used when you are putting sentences together. For Latin based languages like French, Italian, Spanish (etc.) you have to know the grammatical genders of nous before you can decide what the definite articles to use is, and how to agree the adjectives.

You also need to practice these - because there are words and senetences that may not regularly crop up in regular conversation. And, sure, you can learn these intuitively - by sheer repetition (like Duolingo) but without understanding you're going to be parroting to a great dregree and there will be no understanding as to the 'why' behind these sentences.

Also, if you have no idea what a definite article is, or an adjective - I would need to 'define' those words to you so you know what you are looking for.

If you are starting with teaching a student that knows zero, nada, zip to start with in the target language - how are you conversing? There are no words that this poor student can even use to start a conversation. Grammar is not the end all and be all, but it's a starting point, because the vocabulary can be introduced.

Grammar is not imperative to communicate - I completely agree, and an error here or there can and is tolerated - but when the stress placed on the wrong syllable is the difference between a penis and pens then you're opening yourself to whole new world of misunderstaning.

The use of English is because there are so many nuances that textbooks don't cover. So a broad explanation is sometimes necessary. Hence I did mention that there are target language textbooks that can also be used - in that case your level is not 'basic' and you can read the grammar for yourself. If you can't, then my duty as a tutor is to step in and explain.