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～ことにする　ｖｓ　～ことにした
～と思う　ｖｓ　～と思った

What sort of context would you use either of these? Does the past tense imply that you've changed your decisions or thoughts?

chocolate
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bvblue
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    Although I think I have an idea, it would be helpful if you could list specific cases of what you're unsure of. – dainichi Oct 15 '13 at 09:16
  • I didn't really have any specific examples to start off with. But just say something like: 明日、デパートに行くことにする　ｖｓ　明日、デパートに行くことにした for "I've decided(?) to go to the department store tomorrow" – bvblue Oct 20 '13 at 11:11

1 Answers1

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You have actually picked two good examples to explain an odd corner of Japanese. 〜と思う and 〜ことにする both are "state-change" verbs regarding things that happen in people's heads. These sorts of verbs have rules.

〜と思う

  1. Plain: State-change

    （私は）ジョンが大丈夫だと思う。
    Lit. "I just had the thought that Jon is okay."
    "I think that Jon is okay."

  2. 〜ている: Stative

    （私は）ジョンが大丈夫だと思っている。
    "I think that Jon is okay."

  3. 〜た: Past state-change

    （私は）ジョンが大丈夫だと思った。
    Reading 1: "I think that Jon is okay (but perhaps he's not?)."
    Reading 2: "I thought that Jon was okay (and no longer do)."

    (Reading 1 is more likely unless context suggests 2.)

  4. 〜ていた: Past stative

    （私は）ジョンが大丈夫だと思っていた。
    "I thought that Jon was okay (but no longer do)."

All of the above sentences are just fine with the listed readings. However, note the following:

ｘ彼は、ジョンが大丈夫だと思う。
Lit. "He just had the thought that Jon is okay."

You cannot say this because it suggests that you can essentially see someone's mind change on the fly. You need to instead use one of the other forms, like 思っている or 思った. (Or wrap it in say, でしょう.)

〜ことにする

This behaves essentially like 〜思う.

  1. Plain: State-change

    （私は）明日デパートに行くことにする。
    Reading: "I (just) decided to go to the department store tomorrow."

  2. 〜ている: Stative

    (私は）明日デパートに行くことにしている。
    Lit. "? I am decided to go to the department store tomorrow."
    Reading: "I intend to go to the department store tomorrow."

  3. 〜た: Past state-change

    (私は）明日デパートに行くことにした。
    Lit. "I decided to go to the department store tomorrow."
    Reading: "(A little while ago), I decided to go to the department store tomorrow."

  4. 〜ていた: Past stative

    （私は）明日デパートに行くことにしていた。
    Lit. "? I was decided on going to the department store tomorrow."
    Reading: "I was intending on going to the department store (but now I'm not)."

    This form isn't really used. It would be much more common to use 〜予定だった or similar.

Just as with 〜と思う, you cannot use the plain 〜ことにする form with another person, because it suggests you can see inside their mind.

Darius Jahandarie
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  • +1 for the nice explanation. Any reason you say ジョンが instead of ジョンは, though? The more natural choice (TBH, it is THE right choice) is using は there when there is only one sentence without further context, and you need a reason if you use が. It needs a particular context that calls for が. –  Dec 06 '13 at 07:06
  • @TokyoNagoya Thanks. I made that decision because if you actually add the 私は, which I wanted there for disambiguation, then ジョンは sounds weird (contrastive), so I went with ジョンが. As you say, this is more natural: ジョンは[大丈夫だと]思った。 Lit. "As for John, (I) thought he was okay." which unfortunately also has the other meaning Lit. "As for John, he thought it was okay." which I think would be particularly confusing to learners in these examples. – Darius Jahandarie Dec 06 '13 at 16:18
  • Understood. You think of everything! –  Dec 06 '13 at 22:34