When to use DE

Question

How do l know when to use de in the sense of some although it isn't necessarily expressed in the English sentence eg je mange DE la confiture = l eat /am eating jam or perhaps peu d'élèves ?

Avery Enns · Accepted Answer

Hi Myna, 
Here's a link to the lesson which I think is most relevant to your question:
 https://progress.lawlessfrench.com/revision/grammar/du-de-la-de-l-are-used-to-express-some-or-any-partitive-articles 
There are many questions (and answers) in the discussion section of the lesson if you want further info. 
As far as the example sentences you gave, yes, the first one denotes that you are eating some jam. But as for your second example, "peu de" (or "peu d'" in front of a vowel or mute h) actually means not many or few, which has a different meaning than some as it emphasizes that there is a low quantity of whatever noun is being discussed (e.g. "few/not many students). I hope this helps!