What is the difference between 大事 (daiji) and 大切 (taisetsu)?

Question

These are two words that seem like they are basically interchangeable at most levels, as they generally are. The difference between them appears to be small, but what is the difference, if one exists? I feel like there are certain situations in which you would use 大事 and certain situations in which you would use 大切. For example, I would associate 大切 with sentimental things because of the 切 in it that comes from 切ない, things like memories, or a present that someone gave you, whereas I would associate 大事 with things that are of pressing importance, closer to 重要. But this does not appear to be the case, as I occasionally see 大切 used where I would normally expect 大事, like 大切なお知らせ, with more of a feeling of 緊急 or urgency. There are sources here and here which address this, but they both arrive at the same wiggly uncertain feeling that 大切 is more emotional and 大事 is more about urgency while admitting that in just about any situation the two can be used interchangeably.

大辞林 isn't much help on the subject. For both it says 重要であるさま, and in 大事 it includes 大切に扱う and in 大切 it includes 大事にする. The only hint I can get from the dictionary entry is in the following for 大切:

大切

丁寧に扱って、大事にするさま。「本を―にする」「命を―にする」

And in the following for 大事:

大事

重要で欠くことのできないさま。ある物事の存否にかかわるさま。「―な用を忘れていた」「今が―な時期だ」

This lends something to the idea of 大切 as a more 'polite' form of importance while 大事 is more about the concept that something is fundamentally necessary and in that regard important. However both of the entries generally seem to be nearly the same with different wording.

Is there a practical difference, and if so, what is it? Does using one over the other have any impact on the actual meaning of a sentence? When can they not be used interchangeably (outside of set phrases like お大事に)?

Answer 1

As far as I can tell, you've basically hit the nail on the head with the differences between the two.

大切 is listed in the 日本国語大辞典 as (I've removed some of definitions I think are less important):

...
2: 一番必要で、重んずべき物であること。貴重であること。肝要であること。
...
4: 心をくばってていねいに取り扱うこと。大事にすること。かけがえのないものとして心から愛するさま。

So using definition 2, something which is of utmost importance/precious/essential and should be given great regard to, which I see in reference to things which are done (which I think would e.g. be used like 体を大事にするのが大切です, though 大切/大事 could be exchanged there as they can be very similar).

And using definition 4 is with the emphasis on the "treat something mindfully/carefully" and "love something from the bottom of one's heart (心) as if there's no substitute" which I see more for describing things/people as in 大切な（物・者） (though it's also listed as "treated as 大事").

---

And for 大事 (as an adjective):

1: かけがえのないものとして大切にするさま。大切。
2: 評価して心にとめるべきさま。重要で根本にかかわるさま。

So it's used for things which are to be treated as 大切 as if they can't be replaced, or a state where something should be valued and given heed to, so there's a lot of overlap, but I believe that 大切 is more about the 心/sentimental/emotional sense and is more subjective and 大事 more about being fundamentally important/valuable and is more objective.

Note that 大事 can also be used as a noun to mean an important task/large undertaking (as in 大事業) or an important thing etc.

Answer 2

If one look at the kanji he should say there's no big difference between them.
If a difference should be pointed out is in the kaji 切.
大事 daiji is "a big fact", i.e. a fact of great importance.
大切 taisetsu uses the kanji of kiru to emphasize 大. Much like it happens in 親切 shinsetsu, 痛切 tsuusetsu, 哀切 aisetsu, 適切 tekisetsu, 懇切 konsetsu.
So, if we look at the kanji, we have "overtly big" or "extraordinarily big".

Setting aside the emphasis there shouldn't be a real difference, however the use of taisetsu is narrower.
It's surely true that you could say "jaa, sore wo taisetsu ni ne", while you would say "o-daiji ni", but this doesn't mean you'll use daiji in formal context only, and one can safely say "soredewa o-karada wo otaisetsu ni", much like he does using daiji.
Then the difference isn't in the formalilty or in the politeness (but I'll talk later about this).
If you try to google "大事なポイント" or "大切なポイント" you'll get the same number of results, more or less, and you'll see similar contexts.

When the context allows it, one can translate daiji as "important" or "relevant" and taisetsu as "precious".
On the other hand, if precious is "too much" for the context, then we can see/feel that taisetsu is used usually to point out that the noun (the noun taisetsu is referred to) should be FELT as important (precious, grave, really serious...).

And so (IF we want to find a difference between daiji/taisetsu na pointo) we can say daiji na pointo speaking about "the key-points of a reasoning", while we can say "taisetsu na pointo" if we're talking about the main points the students should (perceive as crucial and thus) remember.
However there are situations where the difference is more obvious...
taisetsu na kodomo = a beloved/dear child (precious to the speaker)
daiji na kodomo = an important child (he's the key to my 人体実験 Bwah-ah-ah!)

A context that involve such a nuance (i.e. the thing should be FELT as important) will usually be a more colloquial context, and this is the reason, imho, taisetsu can be perceived as more informal.

This doesn't mean daiji na hito is different from taisetsu na hito, if I'm talking about someone I love. But if I'm talking about the prime minister and I say "taisetsu na hito" then I'm probably implying he's precious (for my country, for instance).
This is why I said its use is narrower than the one of daiji.

Also, "daiji ni (suru/omou...)" and "taisetsu ni (suru/omou...)" are the same when they are referred to the idea of "treating something with care" (see the definition from daijirin).

Answer 3

Very roughly, you could say 大事 translates to "important", while 大切 translates to "precious". For example, you might say "Money is important", but your child or family heirloom is precious.

That being said, 大事 and 大切 are closer together in usage than important vs. precious, to the point that two people i just asked in the workplace flatly said "there is no difference", until they thought about it for a few minutes and though of examples where they would/wouldn't use one or the other word.

Answer 4

大事 is more formal and 大切 is less formal.