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

> 誰かの家に招待されたときは、何か手土産をもっていくものだ  
>  '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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2012-08-02T14:52:31.150
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2015-11-30T15:16:19.370
2015-11-30T15:16:19.370
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question
usage syntax word-choice
Difference between sentence final ものだ and ことだ
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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2012-08-02T23:14:01.027
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2012-08-02T23:14:01.027
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answer
～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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2012-09-04T14:11:17.533
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answer