Using "avec" or "de" to say with in French

Question

The proper use of prepositions is quirky and hard to master. In the Lumberjack Part 1 writing exercise, the phrase in English is, "he was filling up his sleigh with branches". I thought "avec" was too literal and did not sound "French". I wrote, "Il remplissait son traîneau des branches" because "de" can mean "with" and there were multiple branches.

The correct answers are: "remplissait avec des branches" or "remplissait de branches".

Can someone please explain why "de" becomes the plural "des" after "avec" but is the singular "de" when used alone? Thanks.

PS - I think a unit devoted just to the quirks of prepositions would be helpful as well as a vocabulary unit with all the everyday idioms that use etre, avoir, pouvoir, faire, mettre, tenir together with y or en in sentences like "je n'y suis pour rien" or "je n'en peux plus." It's the little simple words that sound the most "French" but are also the most difficult to master. Thanks again.

Accepted Answer

Hi Sean,
I believe it is because it is the verb,
remplir de which means to fill with.
So you can -
remplir de bois, de neige, de charbon, d'allumettes, de branches, de mannequins, etc.
The 'de' will not change.
Not to be confused with 'remplir' (to fill in/ to complete) - 
J'ai rempli des formulaires de toutes sortes = I have filled in forms of all types
Hope this helps!

Answer

The des in "remplissait avec des branches" just means "some". It's plural because branches is plural.In principle, the alternative using remplir de would be "remplissait de des branches", but of course you never say "de des", so it reduces to "de".