But you don't want that gate to close before you get a chance to go through it.

[shootingstar]

(Mrs Elm, the librarian speaking)
This isn't a magic lamp and I am no genie. There is no set number (of lives [added by me]). It could be one. It could be a hundred. But you only have an infinite number of lives to choose from so long as the time in the Midnight Library stays, well, at midnight. Because while it stays at midnight, your life - your root life - is somewhere between life and death. If time moves here, that means something very . . .' She searched for a delicate word. '. . . decisive has happened. Something that razes the Midnight Library to the ground, and takes us with it. And so I would err on the side of caution. I would try to think very keenly about where you want to be. You have clearly made some progress, I can tell. You seem to realise that life could be worth living, if only you found the right one to exist inside. But you don't want that gate to close before you get a chance to go through it.
(The Midnigt Library by Matt Haig, episode God and Other Librarians)

I would like to read the last sentence "But you don't want that gate to close before you get a chance to go through it" as an imperative one. Is this corrext? If not, please explain to me why it isn't and what this sentence is supposed to express. I see it has a declarative form, though. It's describing what you don't want, not commanding anything. Nevertheless, this meaning doesn't work in this context in my opinion.

 

[emsr2d2]

It's not an imperative. It's extremely rare for "want" to be used in the imperative. You can't tell someone to want something. They either want it or they don't! In this context, it's a type of advice (or warning), presented as a statement of fact. Mrs Elm is telling Nora that it would be a bad idea for Nora to allow a door to close before she's been able to go through it.

We use this "You don't want ..." construction fairly regularly, in BrE at least. Let's say someone is walking across an area of land that has a lot of holes in it and isn't really paying attention to where they're putting their feet. Someone else might say to them "Watch where you're going. You don't want to sprain an ankle!"