In short

Glue a second verb straight onto the ます-stem of the first and you get one new verb: 読みます -> 読み始める yomihajimeru (“start reading”), 使います -> 使い切る tsukaikiru (“use up”), 書きます -> 書き直す kakinaosu (“rewrite”). The first verb supplies the action, the second supplies the twist - and only the second one ever conjugates.

The rule

Japanese builds huge numbers of verbs by stacking one verb on another. The joint is always the ます-stem (連用形 ren’yōkei) you already met in lesson 40 - no て, no の, no particle in between.

Step 1 - Take the ます-stem of V1. Say the ます-form and delete ます: 読みます -> 読み, 食べます -> 食べ, 降ります -> 降り, 勉強します -> 勉強し.

Step 2 - Attach V2 in its dictionary form. 読み + 始める -> 読み始める. The result is a single word, written without a space and listed in dictionaries as one entry.

Step 3 - Conjugate V2 and nothing else. V1 is frozen. 読み始めます, 読み始めました, 読み始めない, 読み始めて - every ending sits on 始める. V2 also decides the compound’s verb group: 始める is 一段, so 読み始める is 一段; 出す is 五段, so 降り出す is 五段 (降り出して, not ×降り出って). A few V2s also come as a transitive／intransitive pair (続ける／続く, 終える／終わる); there you have to pick the member your sentence needs: 本を読み続けます takes the transitive 続ける, because intransitive 続く cannot govern を.

Phase: when the action starts, runs and stops.

• 〜始める 〜hajimeru - a neutral, usually deliberate or gradual beginning: 習い始める naraihajimeru (“start learning”), 読み始める.
• 〜出す 〜dasu - the action bursts out, suddenly and typically beyond anyone’s control: 泣き出す nakidasu (“burst into tears”), 降り出す furidasu (“start to rain”), 笑い出す waraidasu (“burst out laughing”). Because it reports a surprise, 〜出す is out of place in plans, requests and 〜ましょう.
• 〜続ける 〜tsuzukeru (transitive, 一段) / 〜続く 〜tsuzuku (intransitive, 五段) - the action goes on: 本を読み続けます, 雨が降り続きます.
• 〜終わる 〜owaru (五段) / 〜終える 〜oeru (一段) - the action is finished: 読み終わる, slightly more formal 読み終える. Note that 終わる is mainly intransitive but carries a transitive sense of its own, so 本を読み終わりました with を is completely normal.
• 〜かける 〜kakeru (一段) - “be on the point of” or “half-done and interrupted”: 死にかける shinikakeru (“nearly die”), 言いかけた iikaketa (“started to say and broke off”). Its intransitive partner 〜かかる 〜kakaru (五段) goes on intransitive verbs, for something on the verge of happening by itself: 日が沈みかかっています shizumikakatte imasu (“the sun is about to set”), 溺れかかる, 崩れかかる. The stem gives a handy noun 〜かけ: 食べかけのパン tabekake no pan (“a half-eaten piece of bread”).

Completion and exhaustion.

• 〜切る 〜kiru (五段) - do it completely, to the last drop: 使い切る tsukaikiru (“use up”), 売り切る (“sell out”). With states it means “utterly”: 疲れ切る tsukarekiru (“be worn out”).
• Its potential 〜切れる 〜kireru, and above all the negative 〜切れない, is the everyday form: 食べきれません tabekiremasen (“I can’t finish it all”), 数えきれない kazoekirenai (“countless”).
• 〜抜く 〜nuku - carry it through against resistance: やり抜く yarinuku (“see it through”), 生き抜く ikinuku (“survive”), 考え抜く (“think through”).
• 〜通す 〜tōsu - keep it up from start to finish: 読み通す yomitōsu (“read right through”).
• 〜尽くす 〜tsukusu - do it until there is nothing left: 食べ尽くす tabetsukusu (“eat every last bit”), 知り尽くす shiritsukusu (“know inside out”).

Repetition and correction.

• 〜直す 〜naosu - do it again in order to put it right: 書き直す kakinaosu (“rewrite”), やり直す yarinaosu (“redo”), 考え直す (“reconsider”).
• 〜返す 〜kaesu - do it back, in return or over again: 言い返す iikaesu (“answer back”), 読み返す yomikaesu (“read over again”).

Reciprocal 〜合う 〜au - two or more people do it to each other: 話し合う hanashiau (“talk it over”), 助け合う tasukeau (“help one another”), 知り合う (“get to know each other” - hence 知り合い, “an acquaintance”).

Intensity, direction and manner.

• 〜込む 〜komu - inward, or deeply: physically 飛び込む tobikomu (“jump in”), 書き込む kakikomu (“write in, fill in”); mentally 考え込む kangaekomu (“sink into thought”), 話し込む (“get deep into a conversation”).
• 〜回る 〜mawaru - all over the place: 走り回る hashirimawaru (“run around”), 探し回る (“search high and low”).
• 〜過ぎる belongs to this family too, but it has its own lesson: see lesson 40.
• 〜出す has a second, literal job - “take out”: 取り出す toridasu (“take out, extract”). Same shape, different meaning; context separates them.

Productive or lexicalized - check before you coin. Some V2s are genuinely productive: 〜始める, 〜続ける, 〜終わる, 〜直す and 〜すぎる attach to almost any verb, and you may build them freely. Others (〜込む, 〜抜く, 〜出す as “extract”, 〜切る in 疲れ切る) are frozen into fixed vocabulary items with meanings you cannot always predict - 落ち込む ochikomu is first of all “feel depressed”, and only rarely the literal “fall into” its parts suggest. Treat those as words to look up, not as a formula.

This is not the て-form family. Lesson 39’s 〜ていく／〜てくる and lesson 36’s 〜てしまう join at the て-form and add grammar (aspect, direction, regret). Stem compounds join at the stem and make new vocabulary. That is why they stack happily: 材料を使い切ってしまいました tsukaikitte shimaimashita (“I’ve gone and used up all the ingredients”).

Forms & patterns

The join is always the stem, and V2 carries everything.

What each V2 adds.

Paired V2s: pick the member your sentence needs.

Two different joints, two different jobs.

Examples

Common mistakes

• 本を読む始めました。 (Hon o yomu hajimemashita.) -> 本を読み始めました。 (Hon o yomihajimemashita.) - the joint is the ます-stem, never the dictionary form. Say the ます-form first, drop ます, then attach.
• 日本語を勉強して始めました。 (Nihongo o benkyō shite hajimemashita.) -> 日本語を勉強し始めました。 (Nihongo o benkyō shihajimemashita.) - the て-form belongs to the auxiliaries 〜てしまう／〜ていく. Lexical compounds take the bare stem: 勉強します -> 勉強し -> 勉強し始める.
• 来月から本を読み出します。 (Raigetsu kara hon o yomidashimasu.) -> 来月から本を読み始めます。 (Raigetsu kara hon o yomihajimemasu.) - 〜出す reports a sudden, uncontrolled onset, so it clashes with a plan. Keep 〜出す for 泣き出す, 降り出す and other surprises.
• 私は日本語を勉強し続きます。 (Watashi wa Nihongo o benkyō shitsuzukimasu.) -> 私は日本語を勉強し続けます。 (Watashi wa Nihongo o benkyō shitsuzukemasu.) - 続く is intransitive and cannot govern を; the transitive partner 続ける is the one you need when a person keeps doing something.
• この量は食べ切りません。 (Kono ryō wa tabekirimasen.) -> この量は食べきれません。 (Kono ryō wa tabekiremasen.) - “cannot finish it all” needs the potential 〜切れる. 食べ切りません would mean you have decided not to finish it.

Tips

• 始める = you decided, 出す = it happened to you. Try inserting 急に kyū ni (“suddenly”): if the sentence still makes sense, 〜出す fits; if you are announcing a plan, giving an instruction or using 〜ましょう, only 〜始める will do.
• Compounds are vocabulary, so use a dictionary. The productive four (〜始める, 〜続ける, 〜終わる, 〜直す) you may build at will. Everything else is safer looked up: 落ち込む is “get depressed”, 取り出す is “take out”, 打ち合わせる is “talk it over in advance, make arrangements” - none of them a sum of their parts.
• Only the last verb inflects - which is also where the group comes from. 読み始める follows 食べる (一段: 読み始めて), while 話し合う follows 買う (五段: 話し合って). When a compound puzzles you, cover V1 with your finger and conjugate what is left.
• Stem compounds stack on top of everything else. A compound verb is just a verb, so it still takes て-auxiliaries, the passive and the potential: 書き直してください, 使い切ってしまいました, 読み終えられませんでした.