In short

The causative 〜（さ）せる covers both English “make someone do it” and “let someone do it” - one form, and the particle on the person decides which: 子供を行かせます kodomo o ikasemasu (“I make the child go”) versus 子供に行かせます kodomo ni ikasemasu (“I let the child go”). Stack the passive on top of it and you get 〜（さ）せられる, the form for “I was made to do something I did not want to do”: 待たされました matasaremashita (“I was kept waiting”).

The rule

Step 1 - Build the causative from the verb group. The groups are the ones from Verb groups and the polite ます-form.

• Group I (五段): change the final -u to -aseru. 書く kaku -> 書かせる kakaseru, 待つ matsu -> 待たせる mataseru, 読む yomu -> 読ませる yomaseru, 帰る kaeru -> 帰らせる kaeraseru. Verbs ending in う take わ, not あ: 買う kau -> 買わせる kawaseru, 手伝う tetsudau -> 手伝わせる tetsudawaseru.
• Group II (一段): drop る and add させる. 食べる taberu -> 食べさせる tabesaseru, 見る miru -> 見させる misaseru.
• する -> させる saseru. Any noun + する verb comes along free: 勉強する -> 勉強させる benkyō saseru.
• 来る -> 来させる kosaseru. The reading shifts to ko-, exactly as in 来ない konai.

Step 2 - Every causative verb is a Group II verb. Once it ends in 〜せる it conjugates like 食べる, whatever the original verb was: 書かせます, 書かせません, 書かせた, 書かせて, 書かせない. One paradigm covers the lot.

Step 3 - Choose を or に on the person, and that choice carries the meaning. This is the whole difficulty of the causative, and it works differently depending on whether the verb already has an object.

With an intransitive verb (行く, 来る, 座る, 休む, 泣く) the を slot is empty, so it is free to mark the person - and the choice is meaningful:

• N1は + N2を + V-causative = make. The causee had no say. 子供を行かせました Kodomo o ikasemashita. (“I made the child go.”)
• N1は + N2に + V-causative = let. The causee wanted to, and you permitted it. 子供に行かせました Kodomo ni ikasemashita. (“I let the child go.”)

With a transitive verb the を slot is already taken by the object, and a Japanese clause cannot hold two を. So the causee is always に, and the make/let distinction is no longer visible in the particles:

子供に野菜を食べさせます。 Kodomo ni yasai o tabesasemasu. - “I make / let the child eat vegetables.” Only context decides.

Step 4 - Use the benefactive verbs to make “let” explicit. When the particles cannot disambiguate, Japanese reaches for the giving-and-receiving verbs from Giving and receiving, plain and benefactive.

• 〜させてあげる - “kindly let someone do it”, from the permitter’s side. 子供に好きな本を読ませてあげました。 As with plain 〜てあげる, saying this to the person you are permitting sounds patronising.
• 〜させてもらう - “be allowed to do it”, from your own side. This is the polite workhorse: 早く帰らせてもらいました Hayaku kaerasete moraimashita. (“I was allowed to leave early.”)
• 〜させていただく - the humble version of the same thing, and by far the most frequent causative in business Japanese. Often it is not a real request for permission at all, just a very deferential way of announcing your own action: 明日、資料を送らせていただきます。 Full treatment is in Keigo: humble forms (kenjougo).

Step 5 - Ask permission with 〜させてください. Literally “please let me do it” - the standard way to volunteer or to ask for leave to act: 少し休ませてください。 Sukoshi yasumasete kudasai. The politer ladder runs 〜させてもらえませんか -> 〜させていただけませんか, built by the same stacking you learned for requests.

Step 6 - Emotive causatives: the person is affected, so を. With feeling verbs the feeler is the subject of the plain verb, so “make someone feel it” has to be built with the causative: 泣く is “cry”, and “make cry” is 泣かせる. Dictionaries list these causatives as transitive verbs in their own right, and the person who feels it takes を even though the plain verb is intransitive - the permissive “let” reading that に carries is not what they express.

Step 7 - The causative-passive: causative + passive = “I was made to”. Take the causative stem and add the passive ending: 書かせる -> 書かせられる kakaserareru. It says that someone else’s causation landed on you, and it carries a clear note of reluctance - you did it, but not because you wanted to.

The case frame is N1は + N2に + 〜（さ）せられる: the person who did the action is the subject, and the person who forced it takes に.

学生は先生に作文を書かされました。 Gakusei wa sensei ni sakubun o kakasaremashita. - “The students were made to write an essay by the teacher.”

Step 8 - The Group I contraction 〜される is what you will actually hear. For Group I verbs, 〜せられる contracts to 〜される: 書かせられる -> 書かされる kakasareru, 待たせられる -> 待たされる matasareru, 買わせられる -> 買わされる kawasareru. The short form is the normal spoken choice and is fully standard.

The one exception: verbs whose dictionary form ends in す cannot contract, because the result would collide with itself. 話す gives 話させられる hanasaserareru only - ×話さされる does not exist. Group II verbs, する and 来る never contract either: 食べさせられる, させられる, 来させられる kosaserareru.

Forms & patterns

Deriving both forms. Romaji in italics; the last column is the contracted spoken causative-passive.

Make or let - reading the particles.

The four voices side by side - one verb, 書く, so you can see the frames move.

Read the last two as a pair: they describe the same event, once from the teacher’s side and once from the student’s.

Emotive causatives - the causative is the only way to say “make someone feel it”, and it is a transitive verb taking を.

Examples

Common mistakes

• 子供を野菜を食べさせました。 (Kodomo o yasai o tabesasemashita.) -> 子供に野菜を食べさせました。 (Kodomo ni yasai o tabesasemashita.) - one clause cannot contain two を. When the verb is transitive, the object keeps を and the causee is forced into に.
• 私は兄に手伝わされました。 (Watashi wa ani ni tetsudawasaremashita.) -> 私は兄に手伝わせました。 (Watashi wa ani ni tetsudawasemashita.) - if you meant “I made my brother help me”, you need the plain causative. 〜させられる points the causation at the subject, so the first sentence says your brother forced you.
• 飲み物を買あせました。 (Nomimono o kaasemashita.) -> 飲み物を買わせました。 (Nomimono o kawasemashita.) - Group I verbs ending in う insert わ, never あ. Same rule as the ない-form 買わない.
• 毎日きさせられました。 (Mainichi kisaseraremashita.) -> 毎日来させられました。 (Mainichi kosaseraremashita.) - 来る keeps the ko- reading right through the causative and causative-passive, exactly as in 来ない konai.
• 会議で話さされました。 (Kaigi de hanasasaremashita.) -> 会議で話させられました。 (Kaigi de hanasaseraremashita.) - the short 〜される contraction is blocked for verbs whose dictionary form ends in す. 話す has only the long form.

Tips

• The particle is the meaning, so decide the particle first. With an intransitive verb, ask yourself: did the person want to do it? If yes, に (let); if no, を (make). If the verb already has a を-object, stop asking - the causee is に and you add 〜てあげる or 〜てもらう when the “let” reading has to be unmistakable.
• 見せる is not the causative of 見る. 見せる miseru (“show”) is its own verb; 見させる misaseru is “make/let someone look”. The same trap sits in 着せる (“dress someone”) versus 着させる. A short-looking form is often a separate lexical verb, not a derivation.
• Where English says “I got to”, Japanese does not use the causative-passive. 〜させられる always carries reluctance. “I got to try it” is 試させてもらいました, not ×試させられました - the second one says you were forced. Choose 〜させてもらう for anything you were glad about.
• 〜させていただきます is rarely about permission. In shops, e-mails and announcements it is simply a very humble “I will do X” (確認させていただきます, “I will check”). Recognise it as fixed politeness rather than parsing out who is permitting whom.