de + les , only sometimes... why?

Question

Questions about this topic, using the lesson examples:

Il a mangé de magnifiques gâteaux

He ate some magnificent cakes.

J'achète de beaux draps

I buy nice sheets.

Note that when the adjective is placed BEFORE a plural noun, the partitive article des (some) becomes de (or d' in front of a vowel or mute h).

ATTENTION:
This rule doesn't apply when des is the contraction of "de
+ les" (= of/from/to the) :

J'ai acheté de nouvelles bottes

I bought [some] new boots.

My question is:  how is
the 3rd example actually different from the previous two? How do we
know that it would have be “de + les” and that they would not? Why wouldn’t
they also have that option?

Merci  à l’avance!

Accepted Answer

"how is the 3rd example actually different from the previous two?"
J'ai acheté de nouvelles bottes: This example is the same as the previous examples in the lesson it is just used to contrast it with the following example in the lesson which highlights an example when des does not become de:
Je suis jalouse des nouvelles bottes que tu as achetées.
"How do we know that it would have be “de + les” and that they would not? Why wouldn’t they also have that option?"
In the example where des does not change to de you have got the adjective "jalouse" which requires the preposition de. So you know that des cannot be just an indefinite article/partitif (see page linked by Jim) but must be a contraction of de+les meaning I am jealous of the shoes you bought (not some shoes).
Ok, you might now ask: Assuming I want to use a phrase requiring de followed by an indefinite amount. Why is that then not e.g. "J'ai besoin des nouvelles bottes" but "J'ai besoin de nouvelles bottes" (I need some new shoes)?
Here the rule is that the indefinite article is omitted. See also https://www.lawlessfrench.com/grammar/de-vs-du-de-la-des-quantity/.
Bottom line:
While this may all sound very confusing, the basic rule is quite simple: The plural indefinite article/partitif des becomes de when followed by an adjective.

Answer

Salut Robyn,This area of French grammar is very tricky and can be confusing.I suggest that you will find the following link helpful in your overall understanding:-https://www.lawlessfrench.com/grammar/articles/?_rt=MXwxfGFydGljbGVzfDE3MDg0OTYzMTc&_rt_nonce=a3625d4bb1Bonne continuationJim