How to remember genki grammar

I am currently on lesson 11 of genki, but it’s easy for me to forget content in other lessons. For example, in lesson six there is ではいけません, which I forgot with no exercises to reinforce it.

What’s the best way to remember?

What about the workbook, does it not have any tasks in it for that either?

I assume you mean -て-form + はいけません.

Exercise V in lesson 6 (pp. 160) in the textbook covers this (if you have the 3rd edition, at least).

In the workbook, it’s exercise 6 on page 65.

Generally, it’s normal to forget some details about grammar after a while. Seeing the grammar being used helps reinforcing it, the readings in Genki are good for that I think, as well as any other materials you may find (if you understand them).

The てはいけません pattern is not the most commonly used, since you’re only using it when you’re in a position of authority. So it won’t show up as much as other ways of saying similar things (e.g. ないでください). That said, once you’ve seen a bit more Japanese (for example, the potential form), the meaning becomes almost “obvious”, as it translates literally to something like “doing X isn’t able to go (i.e. work out)”.