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Plans and Decisions
Express intentions, make personal decisions, and describe shared arrangements.
～つもり
Use a dictionary-form verb before つもりです to express an intention or plan you have formed. Use a verb ending in ない for an intention not to do something. The plain ending is つもりだ. This pattern describes your intended action, not something you have already completed.
Polite: 今度の連休は、家で古い写真を整理するつもりです。
Plain: 今度の連休は、家で古い写真を整理するつもりだ。
I intend to organize old photographs at home over the next long weekend.
つもり describes an intention, not a guarantee that circumstances cannot change; たい simply expresses what you want to do.
～ことにする
Use a dictionary-form verb or a verb ending in ない before ことにする to describe a personal decision. The past form ことにしました reports that you have made the choice. It can introduce a new routine, but it can also describe a decision about just one occasion.
Polite: 朝の時間を作るために、夜は十一時に寝ることにしました。
Plain: 朝の時間を作るために、夜は十一時に寝ることにした。
I decided to go to bed at eleven at night to make more time in the morning.
ことにした identifies a choice you made; it does not by itself tell us whether you have carried it out yet.
～ことになる
Use a dictionary-form verb or a verb ending in ない before ことになる to report an arrangement or decision as an established outcome. The speaker may have joined the discussion, but the wording emphasizes what has been arranged rather than the speaker’s personal choice. ことになりました reports a new arrangement.
Polite: クラスで話し合って、発表は二人で行うことになりました。
Plain: クラスで話し合って、発表は二人で行うことになった。
After a class discussion, it was decided that we would give presentations in pairs.
Do not translate this as “someone else decided” every time: it can also summarize a group decision you helped make.