how do we know thing are masculine or feminine
la Souris or le Souris
You have to memorize each noun with an article. Sometimes the same word can mean something different depending on the gender.
Le tour = the tour
La tour = the tower
You can also look up a word in the dictionary to see if it's masculine or feminine:
There are noun endings that are often a specific gender.
Some masculine words ending in -eau:
Panneau, Bateau, Cadeau, Plateau
Some feminine words ending in -tion/-sion/ -ion:
Impression, Émotion, Imagination, Condition
Unfortunately, there are always some that aren't the gender that you think:
La peau, L'eau (Fem.), Un avion
Unfortunatelly many languages have grammatical genders.
Mostly they are not logical and you have to learn them together with the noun.
You can read the grammar "Tips and notes" in Duolingo's web version (www.duolingo.com) with your browser in "Desktop mode". They are under the lessons in each skill, that introduces new grammar.
Here is the link to a discussion with very useful comments:
"How do you tell if a sandwich is a boy or girl?"