das die and der
how do you know the difference
The best tip came from Mhre1. It does not really exist any rule to recognize the gender of nouns in German, as for example in Polish. So it helps to learn all nouns always together with their appropriate article. This is important, because of the cases and because you have to conjugate other word types according to the correct gender.
Sadly, if you want to learn some other languages like Spanish, French or Italian, you have to do the same again, because the gender of the nouns differs often between all these languages.
They need to be memorised. Memrise is better for cramming and complements Duolingo very well in this regard, imho.
Furthermore, You can use the following list of word endings and their associated gender as a rough guide:
masculine: -ig , -ling, -or, -ismus
Feminine: -ei, -keit, -ung, -tät, -ik, -heit, -schaft, -ion
neuter: -um, -lein, -tum, -ma, -ment, -chen
This list itself can be pressed into three Mnemonics. Memorise these three words, and you should be fine most of the times:
Of course, being mnemonics, these make no sense otherwise - Though I didn't check the Nachtigaller Dictionary.