"It will help you breathe easy." That's what I saw in Boots. But 2easy2 is an adjective, shouldn't I say EASILY instead of easy? Or can it be both an adjective and an adverb? Thank you.
"Breathe easy" has an idiomatic usage meaning to not feel stress, to not worry, to feel relaxed.
If you wanted a more literal meaning of 'breathing without difficulty' (from congestion, for example), "breathe easily" would be the better choice.
Depending on the context where you heard it, it could mean either or both. Advertisers for cough/cold/allergy medications commonly use this idiom, meaning both that their product helps your breathing, and that you don't need to feel stressed about being sick after you take their product.
(not a teacher, just a language lover)
