Generally, "En" is the pronoun to substitute for "de" objects, like "du lait" or "de la confiture". "Les" is the pronoun for direct objects, i.e. ones you would use "le" or "une" with like "le cadeaux". To be more specific, "en" is for vague quantities and "les" for specific quantities.
Therefore, "J'en prépare" translates to "I prepared some", while "Je les prépare" translates to "I prepared it/them". Basically the same difference as "des" vs "les".
E.g:
"They wanted cake, so I prepared some." translates to "Ils voulaient du gâteau, alor j'en ai prépare."
"We need a dozen traps and I am preparing them (all)." translates to "Nous avons besoin de douze pièges et je suis en train de les prépare."
"(You/you guys) Eat it but leave some." translates to "Mangez-le mais laissez-m'en un peu."
(sorry in advance for any mistakes, grammar ain't my forte)
Generally, "En" is the pronoun to substitute for "de" objects, like "du lait" or "de la confiture". "Les" is the pronoun for direct objects, i.e. ones you would use "le" or "une" with like "le cadeaux". To be more specific, "en" is for vague quantities and "les" for specific quantities. Therefore, "J'en prépare" translates to "I prepared some", while "Je les prépare" translates to "I prepared it/them". Basically the same difference as "des" vs "les". E.g: "They wanted cake, so I prepared some." translates to "Ils voulaient du gâteau, alor j'en ai prépare." "We need a dozen traps and I am preparing them (all)." translates to "Nous avons besoin de douze pièges et je suis en train de les prépare." "(You/you guys) Eat it but leave some." translates to "Mangez-le mais laissez-m'en un peu." (sorry in advance for any mistakes, grammar ain't my forte)
En was vexing me for so long, and now my trick is to say "any" for "en." It's how we would use any.