지금 and 이제 are close in meaning but can have different nuance and usages in some contexts. 이만 has a small overlap with 이제, but it is also quite different. * 지금(adv, n) - now, at this moment - describes something currently happening. * 이제(adv) - now (with the nuance of being different from how it has been so far). "now that ..." or "from now on". * 이만(adv) - "with this" (rather than going further or doing more). It's a contraction of 이만하고, which is an adverb-like form of 이만하다(v), short for 이만큼만 하다 (do this much and no more, or "stop at this point"). It's a phrase based on "this", just as 그만하다, 그만하고, 그만 are based on "that". (이만하다 has another meaning which is 이것만 하다 (이것만큼 하다/크다) "about as big as this") Examples (in these examples, 지금 and 이제 are not interchangeable): - 너 지금 뭐 하니? What are you doing now. - 너 이제 어떻게 할 거니? What are you going to do now [from now on]? - 이제(는) 그런 실수는 다시 안 한다 - Now [from now on], I'll never make that mistake. - 이만 [이만하고] 끝마칩시다 - Let's wrap it up now [at this point]. - 강아지가 송아지만 해요 = 강아지가 송아지만큼 커요 = A/This puppy is as big as a calf.

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