J'ai encore des cadeaux à acheter

Question

Shouldn't this be:
Marie "EN" a déjà tous achetés.  
Since the article here is "des" and not "les"... We just know there are "some" presents to buy, not any specific ones.

Accepted Answer

Bonjour Jonathan !
You can only use "en" when you're replacing a group introduced by de, du, de la or des.
See also https://french.kwiziq.com/revision/grammar/the-adverbial-pronoun-en-can-replace-a-phrase-introduced-by-de
In this sentence - Marie les a déjà tous achetés. - the presence of "tous" means that the original sentence would be :Marie a déjà acheté tous les cadeaux.So no "des" here, therefore "en" is not an option in this sentence, only the direct object pronoun les is correct here.  
 
I hope that's helpful!Bonne journée !

Answer

I Jonathan,
Grammatically, both possibilities are correct, although they have different connotations:
Marie en a déjà tous achetés. Versus: Marie les a déjà tous achetés.
The first one (with "en") implies that there is a certain fixed set of presents of which Marie bought all. The second one makes no such implication. I guess you would account for that in English like this, somehow:
Marie en a déjà achetés. -- Marie already bought all of them.Marie les a déjà achetés. -- Marie bought them all.
I hope that helps, -- Chris (not a native speaker).