'Make / let X do Y' — one form, two readings
The causative says A makes / lets B do something. Forms: る-verbs drop る and add 〜させる (食べる → 食べさせる). う-verbs shift the final u-row sound to its a-row partner and add 〜せる (書く → 書かせる, 読む → 読ませる, 言う → 言わせる). Irregulars: する → させる, 来る → 来させる (read こさせる). The causative form is itself a る-verb and conjugates normally (食べさせる → 食べさせます, 食べさせない, 食べさせた, 食べさせて). Two meanings sit inside one form: 'force / make' (against will) and 'allow / let' (with will) — disambiguated by context, the verb, and tone.
Forming the causative — by verb class
る-verbs: drop る, add 〜させる. 食べる → 食べさせる. 見る → 見させる. おしえる → おしえさせる. う-verbs: change the final syllable from the u-row to its a-row partner, then add 〜せる. 書く (ka-ku) → 書かせる (ka-ka-seru). 読む → 読ませる. 話す → 話させる. 行く → 行かせる. 言う → 言わせる (う→わ). 持つ → 持たせる. 歌う → 歌わせる. Irregulars: する → させる. 来る → 来させる (read こさせる).
Two meanings: 'force' vs. 'let'
The same form covers both 'make someone do' and 'let someone do'. Context tells you which. 子ども に やさい を 食べさせました。 could be 'I made the kids eat their vegetables' (force) or 'I let the kids eat the vegetables' (allow), depending on the situation. With actions a person wants to do, the natural reading is 'let / allow'; with actions a person doesn't want, it's 'make / force'. To disambiguate explicitly: 〜たい のに 〜せない ('won't let, even though they want to') or pair with 〜て あげる ('I let them ~ as a favor': 行かせて あげる = 'let them go (as a kindness)').
を vs. に for the causee — transitive vs. intransitive
If the original verb is intransitive (no direct object — 行く, 泣く, すわる, 来る), the causee (the person made/let to do it) takes を. 子ども を 学校 に 行かせました。 = 'I sent / made the child go to school.' 赤ちゃん を 泣かせて しまいました。 = 'I made the baby cry (oh no).' If the original verb is transitive (already takes a を object — 食べる, 読む, 書く), the causee takes に, because を is already busy marking the object. 子ども に やさい を 食べさせました。 = 'I made / let the child eat the vegetables.' 学生 に 本 を 読ませました。 = 'I had the students read the book.' This を/に rule is the most-tested point of the causative.
'Let me ~' — asking permission with 〜させて ください
Hugely useful chunk: 〜させて ください = 'Please let me ~'. Form: causative 〜させる → 〜させて (te-form) + ください. 行かせて ください。 = 'Please let me go.' 休ませて ください。 = 'Please let me rest / take a break / take time off.' やらせて ください。 = 'Please let me do it (give me a chance).' はらわせて ください。 = 'Please let me pay (it's my treat).' しょうかい させて ください。 = 'Please let me introduce ~.' Standard polite request, very natural in business and social contexts.
Common causative-of-emotion verbs
Some verbs are almost always heard in the causative because their plain forms are intransitive feelings and we usually talk about who caused the feeling. 泣く (cry) → 泣かせる ('make ~ cry'). わらう (laugh) → わらわせる ('make ~ laugh'). おこる (get angry) → おこらせる ('make ~ angry'). しんぱい する (worry) → しんぱい させる ('make ~ worry' — ご しんぱい を おかけ しました is the apology version). よろこぶ (be glad) → よろこばせる ('make ~ happy'). These are the everyday causatives — the 'make-them-feel-something' family.
Don't confuse 〜させる with 〜らせる or the passive
Watch the kana. Causative 〜させる ends in せる. Passive 〜られる ends in れる. They look similar and both attach to the same a-row stem of う-verbs (書か〜), but the consonant differs: 書かせる = causative (make write), 書かれる = passive (be written). For る-verbs the ending difference is 〜させる (causative) vs. 〜られる (passive). One last trap: there is no causative 〜らせる for る-verbs (a common learner mistake) — it's 〜させる: 食べさせる ✓, 食べらせる ✗.
