In short

こと, もの and ところ are “formal nouns” (形式名詞 keishiki meishi): words that have lost their dictionary meaning and now just carry grammar. Learn them as three families - こと for decisions and rules (〜ことにします koto ni shimasu, “I’ve decided to”), もの for the way the world is (〜ものです mono desu, “that’s how it goes”), and ところ for what phase an action is in (〜ているところです te iru tokoro desu, “I’m in the middle of it”). All three take the plain form in front.

The rule

Step 1 - Stop translating them. In these patterns こと does not mean “thing”, もの does not mean “object”, ところ does not mean “place”. They are empty scaffolding, and because they are empty they are written in kana, not as 事・物・所. What decides the meaning is the word bolted onto the back: ことにする, ことになる, ものだ, ところだ.

They are all nouns, so the clause in front of them behaves exactly as it does before a real noun (lesson 18): the verb is plain, however polite the end of the sentence is. ×行きますことにしました -> 行くことにしました iku koto ni shimashita. The politeness is spent once, at the very end.

This is not the free nominalizer こと／の of lesson 26. There, こと turned the clause into a plain noun, and with many predicates の was available instead. Here nothing can be swapped and nothing can be dropped - these are frozen idioms that happen to be built out of the same word.

Step 2 - 〜ことにする: I decide. V-plain + ことにします reports a decision you yourself are making. The past 〜ことにしました is the normal form, because by the time you say it the decision is taken: 来年、留学することにしました (Rainen, ryūgaku suru koto ni shimashita) - “I’ve decided to study abroad next year.” The negative decision uses the ない-form: お酒を飲まないことにしました (Osake o nomanai koto ni shimashita) - “I’ve decided not to drink.”

Step 3 - 〜ことになる: it gets decided. Swap する for なる and the agent disappears, exactly as in lesson 28. 〜ことになりました says the outcome came about - a company, a committee, a schedule or simply circumstances settled it: 来週から大阪で働くことになりました (Raishū kara Ōsaka de hataraku koto ni narimashita) - “it’s been arranged that I’ll work in Ōsaka from next week.”

The point learners miss: Japanese prefers ことになる even for your own decisions, when you are announcing them. Saying 結婚することにしました sounds like you are taking credit for the plan; 来月、結婚することになりました (Raigetsu, kekkon suru koto ni narimashita) presents the same news modestly, as something that has come about. For personal news - marriage, a move, leaving a job - なる is the socially safe choice.

ことになる has one side reading worth recognising: “that means, that works out as”. では、全部で五人参加することになりますね。 (Dewa, zenbu de gonin sanka suru koto ni narimasu ne.) - “So that means five people will be taking part in all.”

Step 4 - Add ている and you get rules instead of events. This is a clean four-way system, and the ている is the resultant-state ている of lesson 15: deciding is instantaneous, so 〜ている points at the decision still standing, and that standing decision is what reads as a rule.

• 〜ことにしています - a rule I impose on myself, a personal policy: 毎朝六時に起きることにしています (Maiasa rokuji ni okiru koto ni shite imasu).
• 〜ことになっています - a rule that exists, imposed by an organisation, a law or a schedule: ここでは靴を脱ぐことになっています (Koko de wa kutsu o nugu koto ni natte imasu) - “here you are supposed to take your shoes off.”

Step 5 - Two more こと idioms. V-dictionary + ことはない means “there is no need to” - and note that the negation sits in ない at the end, not on the verb: 心配することはありません (Shinpai suru koto wa arimasen) - “there’s no need to worry.” Interrogative + plain + ことか is exclamatory: it says you have finally recognised the size of something. どんなに心配したことか。 (Donna ni shinpai shita koto ka.) - “How worried I was!”

Step 6 - 〜ものだ: the way things are. もの appeals to shared common sense - not to this situation but to how the world generally works. Three jobs - the た-form marks the second one off, and context separates the other two, which share the dictionary form:

1. General truth (dictionary form): 人はいつか死ぬものです (Hito wa itsuka shinu mono desu) - “people die eventually; that is how it is.” It works with adjectives too: 時間がたつのは早いものです (Jikan ga tatsu no wa hayai mono desu) - “time really does fly.”
2. Nostalgic recollection (た-form): 子供のころは、よくこの川で遊んだものです (Kodomo no koro wa, yoku kono kawa de asonda mono desu) - “as a child I used to play in this river a lot.” This is “used to” with warmth attached; the neutral past would be a bare 遊びました asobimashita, which reports the fact and nothing more.
3. Social advice (dictionary form): 先生には敬語を使うものです (Sensei ni wa keigo o tsukau mono desu) - “you use polite language to a teacher - that’s how it’s done.” Because it invokes a norm rather than your opinion, it is much stronger than 〜たほうがいいです from lesson 35.

The negative 〜ものではありません is the matching reproach: 人の悪口を言うものではありません (Hito no waruguchi o iu mono de wa arimasen) - “one doesn’t speak ill of people.”

Step 7 - 〜ものだから／〜もので: the excuse. Plain + ものだから (shortened to もので) gives a reason while pleading that it was out of your hands. It is the reason particle から of lesson 21 with an apology baked in: すみません、道が混んでいたものですから。 (Sumimasen, michi ga konde ita mono desu kara.) - “Sorry - it’s just that the roads were jammed.” Use it when excusing yourself, not when stating a neutral cause.

Step 8 - 〜ところだ: which phase the action is in. This is the most systematic of the three. The same verb takes three different forms in front of ところ, and each pins the action to a different point on the clock:

  ──[ 食べるところです ]───[ 食べているところです ]───[ 食べたところです ]──>
        about to eat            in the middle              just finished

食べる taberu + ところ = you have not started. 食べている tabete iru + ところ = it is under way. 食べた tabeta + ところ = it finished seconds ago. Adverbs make the phase audible: 今から ima kara / これから kore kara with the dictionary form, 今 ima with ている, たった今 tatta ima with た.

A separate pattern shares the word: V-た + ところ (with no だ) means “upon doing X, I found that…”, and the second clause is a discovery, normally in the past: 電話をしてみたところ、留守でした。 (Denwa o shite mita tokoro, rusu deshita.) - “I tried phoning, and it turned out nobody was in.”

One to recognise only. 〜どころではない dokoro de wa nai (“this is no time for X”) looks related but is a different, emphatic construction and belongs to a later stage. If you see どころ with a voiced do, it is not the phase system.

Forms & patterns

The こと family.

する or なる, one-off or ongoing. Read this grid across for the agent and down for the time span.

The もの family. Note the linkers: な-adjectives keep な, and nouns need な before ものだから.

The ところ phase system, shown on one verb.

〜たところだ versus 〜たばかりだ (lesson 34). Both translate as “just did”, but they measure different things.

Examples

Common mistakes

• 日本へ行きますことにしました。 (Nihon e ikimasu koto ni shimashita.) -> 日本へ行くことにしました。 (Nihon e iku koto ni shimashita.) - こと, もの and ところ are nouns, so the verb in front is plain. The ます-form never goes in front of a noun.
• 転勤することにしました。 (Tenkin suru koto ni shimashita.) -> 転勤することになりました。 (Tenkin suru koto ni narimashita.) - if the company decided, use なる. する claims the decision was yours, which sounds odd about a transfer - and immodest even about a marriage.
• 今、レポートを書くところです。 (Ima, repōto o kaku tokoro desu.) -> 今、レポートを書いているところです。 (Ima, repōto o kaite iru tokoro desu.) - dictionary form + ところ means you have not started yet. For “in the middle of”, ところ needs 〜ている in front of it.
• 毎日走ることになっています。 (Mainichi hashiru koto ni natte imasu.) -> 毎日走ることにしています。 (Mainichi hashiru koto ni shite imasu.) - for a rule you set yourself, にしている. になっている hands the rule to somebody else and would mean running is required of you.
• 心配しないことはありません。 (Shinpai shinai koto wa arimasen.) -> 心配することはありません。 (Shinpai suru koto wa arimasen.) - the negation already lives in ありません. English “no need to worry” feels negative, so learners negate the verb too and end up with a double negative.

Tips

• つもり is a plan, ことにする is the decision itself. 行くつもりです (lesson 38) reports an intention that could still change; 行くことにしました reports the moment you closed the question. If a colleague could still talk you out of it, use つもり.
• なる is politeness, not passivity. The habit of announcing your own news with ことになりました is a genuine social reflex, not vagueness. It lets you share good news without sounding as though you are congratulating yourself, and it is the phrasing you will hear in office announcements and wedding invitations.
• ところ is a clock, ばかり is a feeling. 日本に来たばかりです works three months after you landed; 来たところです would mean you walked through the door as you spoke. When in doubt about how much time has passed, ばかり is the forgiving one.
• Three empty nouns, three questions. Ask “was it decided?” -> こと. “Is this how the world works?” -> もの. “Where in the action are we?” -> ところ. That triage handles almost every occurrence you will meet at this level.