Which words are shortened by an apostrophe in french?

Almost every Word that consists of a consonant sound followed by the letter e is contracted when in front of a Word that begins with a vowel sound.

The only exception is ce (the masculine singular demonstrative adjective), which changes to cet when the next Word starts with a vowel sound.

Here is a full list:

de

je

la (the only one that doesn't end in e)

le

me

ne

que (including Words like lorsque, jusque, quoique, etc.)

se

te

Finally, you have the particular case that si becomes s' when in front of il or ils (and only then).

Examples:

Le Fabuleux Destin d'Amélie Poulin (film title)

J'aime m'habiller en blanc.

Donnez-m'en un, s'il vous plaît.

Quoi qu'il fasse, il n'y arrive pas. Addition by Crisdean

So rare to see such a good answer. But I could not help myself looking for exceptions.

As Billy explained, the coordination conjonctions Words finishing by -que (lorsque, jusque, quoique, etc..) are contracted when they are followed by a Word beginning with a vowel...

except : Presque (almost, nearly)

C'est presque impossible : it's nearly impossible.

The other exception is when Je (first person singular subject pronoun) is inverted in a question, it does not contract. Puis-je avoir de l'eau ? . Can I have some water ?

Everyday, I realise how french can be hard for foreigners.