As the lesson explains an EXCEPTION that -s needs to be added in Tu form of -ER verbs with Adverbial Pronouns. Does that rule also apply in negation? 

- Manges-en trois! [Eat 3 of them] --> N'en manges pas trois! [Don't eat 3 of them!]   ??

- Achètes-en! [Buy some!] --> N'en achètes pas! [Don't buy any!]   ??

- Vas-y! [Go there!]     -->     N’y vas pas! [Don't go there!]   ??

-  Manges-y! [Eat there!]  --> N'y manges pas! [Don't eat there!]    ??