feminine versus masculine
how do you remember which words are feminine and which words are masculine?
That's tough. Nouns that end in e are more likely to be feminine than masculine, but rules about French grammar are complicated because there are so many exceptions and irregularities. You have to learn the same way French children do, repeating them over and over, getting corrected when you get them wrong.
Honestly the best way is just to really try to remember as you go. Gender changes words in a sentence and being off on the gender can make a whole bunch of other words incorrect because they don't match.
I find that there's plenty of opportunity to learn the gender of things because you will get a question wrong if you write the nouns wrong. For example, when I learned the names of clothes I really tried to learn all the genders. There is always going to be some I'm not quite sure of. Later on when I'm describing the colour or who owns them I'm going to be told that I either got the gender right or wrong by if Duo says the rest of my sentence matches it.