Indicates large quantity or frequency
When to use
quantity expression, frequency expression, both positive and negative
The nuance
많이 is used when the quantity or frequency is high; it can be used in both positive and negative contexts.
많이 (man-i, mani) · 너무 (neo-mu, neomu) · 아주 (a-ju, aju)
'많이 (mani)', '너무 (neomu)', and '아주 (aju)' all express degree, but 많이 indicates quantity or frequency, 너무 implies excessive or negative degree, and 아주 conveys a strong positive degree.

One lane per word — no nested boxes, just the gist.
Indicates large quantity or frequency
When to use
quantity expression, frequency expression, both positive and negative
The nuance
많이 is used when the quantity or frequency is high; it can be used in both positive and negative contexts.
Used when the degree is excessive or has a negative nuance
When to use
negative emphasis, excessive degree, negative situations
The nuance
너무 is used to emphasize excessive or negative degree and is unnatural in positive contexts.
Expresses strong positive degree, suitable for moderate praise or emphasis
When to use
positive emphasis, praise expression, moderate degree
The nuance
아주 is used for strong positive emphasis in affirmative contexts and sounds natural without excess.
나는 어제 많이 공부했어요.
na-neun eo-je man-i gong-bu-haess-eo-yo., na neun eo je man i gong bu haess eo yo.
Correct use of 많이 to indicate high frequency or quantity.
나는 어제 너무 공부했어요.
na-neun eo-je neo-mu gong-bu-haess-eo-yo., na neun eo je neo mu gong bu haess eo yo.
너무 properly expresses excessive degree here.
나는 어제 아주 공부했어요.
na-neun eo-je a-ju gong-bu-haess-eo-yo., na neun eo je a ju gong bu haess eo yo.
아주 is unnatural alone with '공부하다' without additional context or adjective.
이 음식이 너무 맛있어요!
i eum-sik-i neo-mu mas-iss-eo-yo!, i eum sik i neo mu mas iss eo yo!
너무 sounds excessive in positive praise; use 아주 or 정말 instead.
이 음식이 아주 맛있어요!
i eum-sik-i a-ju mas-iss-eo-yo!, i eum sik i a ju mas iss eo yo!
아주 fits positive emphasis naturally here.
그는 많이 피곤해 보여요.
geu-neun man-i pi-gon-hae bo-yeo-yo., geu neun man i pi gon hae bo yeo yo.
많이 correctly expresses large degree of tiredness.
그는 너무 피곤해 보여요.
geu-neun neo-mu pi-gon-hae bo-yeo-yo., geu neun neo mu pi gon hae bo yeo yo.
너무 properly emphasizes excessive tiredness.
그는 아주 피곤해 보여요.
geu-neun a-ju pi-gon-hae bo-yeo-yo., geu neun a ju pi gon hae bo yeo yo.
아주 is unnatural with negative states like tiredness.
Which is the most natural expression for positive praise?
Which word expresses an excessive degree?
Which word is appropriate to express large quantity or frequency?
What does it mean?