WildGinger10 wrote:But... but... but... isn't the declension of verbs and adjectives in relation to their nouns and cases super important??
It doesn't mean that the declension rules suddenly don't apply anymore. You just have two correct forms in this case. I guess you could compare it to "you are/you're" in English. Of course that's an entirely different grammar point but it's comparable in the sense that you have two options and both are correct.
WildGinger10 wrote:I mean, you couldn't do this to, say, deine because Augen is plural which makes it feminine (and die Auge is feminine anyway). You can't say Schleiße dein' Augen... can you??
No, you can't say "Schlie
ße dein' Augen". You can, however, say "Schließe
deine Augen" or "Schließ deine Augen". As SGP mentioned, the version without -e sounds a bit less formal and tends to be used more in spoken language.
Auge is neutral not feminine.
In normal everyday language you can only do this to verbs and it doesn't work for all of them:
Bei einer Reihe von Verben ist das Endungs-e allerdings verbindlich, etwa, wenn der Verbstamm auf d/t oder Konsonant plus m/n endet: Atme gleichmäßig! Rechne gefälligst sorgfältiger! Arbeite nicht so viel! Auch bei den Verben, die auf -ern und -eln enden, sind die Formen mit Endungs-e obligatorisch, wobei das e der Bildungssilbe auch wegfallen kann: Hand[e]le endlich einmal selbstständig! Trau[e]re doch nicht um ihn, das war er nicht wert!
Source: https://www.duden.de/sprachwissen/sprac ... Imperativs
If I were you, I wouldn't worry about it too much at this point. It's a minor grammar issue and there is no need to learn all the rules and exceptions for this at the beginning. It's just not worth the effort.
Later on, after you've read a couple more books and seen enough examples, you'll probably get it right instinctively.