It is a gender thing!

A large proportion of people who are studying French cannot get their head around the fact that everything has a gender. "Why is a chair feminine and the armchair masculine? " "Parce que..."(because!). You just have to accept it...
The important things to remember is that

•  it is crucial to learn any new word straight away with its gender and
• not knowing the gender can lead to confusion and to chaotic sentences.

Confusion because some words might sound the same,even are writtten the same way but will mean something different in masculine and feminine. For example:

la cour (the playground) / le cours (the course)
la tour (the tower) / le tour (the trick, the turn)
la livre (the pound) / le livre (the book)
la foi (faith) / le foie (the liver)
and there are dozens more...

Chaotic sentences because the article (a,an,the,this,which,my, etc...) and the adjectives (words which describe the noun: the item or the person) agree with that noun. For example:

La belle vielle chaise (the beautiful old chair)
but
Le beau vieux fauteuil (the beautiful old armchair)
Can you see how the beautiful old is completely different in each case.

The gender of the noun (chair/armchair) has a snowball effect on the rest of the sentence. If you get the gender wrong, you will not make one mistake but 3 mistakes in one go!!
See more on following links:
More words with dual gender
Quiz: 100 French nouns, masculine or feminine?