I don't remember if it has already appeared on the tree but I think that's (for me) the first negative imperative I find in the course. What I want to point out is the third form of negative in Greek, μην, which, if I'm not wrong, is only used in this case (maybe also in conditionals?)
It's also used for "in order not to..." sentences, if I'm not mistaken:
- Κράτα το μην πέσει "Hold it lest it fall / so that it does not fall"
- Το κράτησε για να μην πέσει "He held it so that it would not fall"
I think after να in general you have to use it instead of δεν, e.g. Ήθελε το κορίτσι να μη λέει τίποτα στον παππού "He wanted the girl not to tell the grandfather anything".
The (positive) imperative has a form of its own (well, two, usually: one based on the perfect stem for repeated or continual actions, one based on the aorist stem for punctual actions), but the negative imperative is formed from μη(ν) + subjunctive. (Again, choice of present subjunctive or aorist subjunctive depending on aspect.)
Yes, we have a subjunctive mood to describe situations which are not real. For example:
I wouldn't do that if I were you
If only the guys didn't notice me now
I can't properly explain you the imperative - not so good at teaching (the words change, but the order can be practically anything as in Russian generally), but we have it too, of course