Appelsin comes from Dutch appelsien, which used to mean Chinese apple (Sina=China) after the fruit's country of origin. (Just a fun language fact that might serve as a mnemonic)

I totally thought it IS with a "et" since.. oranges are not alive o-o

Because the gender of the noun is masculine, an -en word.

it's æble, an apple is et æble and THE apple is æblet ;)

Because that's incorrect English.

there is no "a", there is "et" and "en", the "et" mostly goes for things that are NOT alive, to where "en" is for alive objects, but it's Danish, sometimes "en" goes for things that are not alive so you have to learn them by heart ;)