My understanding is that ものだ is used to assert things which are mostly known to everyone.

'When invited to somebody's home it's customary to bring presents'.

On the other hand, ことだ is used to give someone advice.

A: どうすれば日本語が早く簡単に上達しますか。
B: 上手になりたいなら、真面目にコツコツ勉強することです。それ以外に方法はありません。

I can understand that B gives an advise to A, however B is not really saying anything that A couldn't already know. Moreover, when talking about what to do when going to somebody's home, "bring presents" is not necessarily something that everybody is aware of - could be that some cultures are used to bringing money.

So it seems to be that, in the examples above, ものだ and ことだ are can be freely exchanged. Note, that the examples are from my textbook so i guess they can't be exchanged.

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    I would say that it parallels the meanings of 「物」 and 「事」 themselves... – Ignacio Vazquez-Abrams Aug 2 '12 at 15:32
  • I was sure we already had a post about the differences of もの　and こと..I just can't find it. – Chris Aug 2 '12 at 23:49
  • @Chrisさん-- これ？→japanese.stackexchange.com/questions/4225/… – user1016 Aug 3 '12 at 1:19
  • In the first sentence もの can easily be seen as corresponding to とき. Do you have a better example where no explicit noun phrases is in the subject position before? – Derpius Dec 2 '15 at 12:32

I am not sure where your confusion comes from, but …するものだ means “it is customary to do …” and …することだ means “do ….” Their meanings are different, and they are not interchangeable.

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  • So i will just assume that the examples do not explain the difference well enough. – buskila Aug 3 '12 at 4:22
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    @Oleg Levy: I would not blame the examples in this case. The first example has a correct translation as long as ものだ is concerned, and you cannot replace ものだ with ことだ without changing the meaning. In the second example, note that B is not talking about what is customary and what is not; B is just talking about what A should do. If you replace ことだ with ものだ in B’s reply, the dialogue does not make sense. – Tsuyoshi Ito Aug 3 '12 at 4:37
  • In the song 夏夕空, one part of the lyrics match this question exactly. 人として守るもの、人として学ぶこと。Does it mean "as a person, we are obliged to protect; as a person, learn" ? (I'm trying to grasp the nuance of this kind of もの and こと usage) – Flaw Oct 5 '12 at 8:32
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    @Flaw: Isn’t that simply “Things to protect as a human; facts to learn as a human”? I will not claim that my interpretation is the right one, though. – Tsuyoshi Ito Oct 5 '12 at 11:00

～ものだ is used to state something that is common sense, and not the speaker's personal opinion ("one should.."). It is similar to ～すべきだ.

～ことだ is used to give and advice that is based on the speaker's own opinion or judgement. It is similar to ～したほうがいい, and should be avoided when talking to one's superior.

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