N4
せる・させる
To make/let/have (somebody do something)
息子に野菜を食べさせる。
I make my son eat vegetables.
Explanation
「せる・させる」 has 3 uses
1Making someone do
Ordering or telling someone to do something: 「生徒に宿題をやらせる」 make the students do homework.
息子に野菜を食べさせる。
I make my son eat vegetables.
友達にトマトを育てさせる。
I make my friend grow tomatoes.
2Letting someone do
Allowing someone to do what they want: 「子供を自由に遊ばせる」 let the children play freely.
１０年間スポーツをさせたから、うちの子は健康だ。
I made my child do sports for 10 years, so she is healthy.
子供に色々なことをさせる必要がある。
It is necessary to make/let children do many things.
3Causing a feeling
Causing someone to feel something: 「両親を安心させる」 put one's parents at ease, 「妹を泣かせる」 make one's sister cry.
お兄ちゃんが妹を泣かせた。
My older brother made my younger sister cry.
親を心配させることは、してはいけない。
You should not do anything that would make your parents worry.
Mixed up with
Particles: with an intransitive verb the person made to act takes 「を」: 「子供を走らせる」. With a transitive verb it takes 「に」 and the object takes 「を」: 「学生に本を読ませる」. If an intransitive verb already has 「を」 (「道を歩く」), the person takes 「に」: 「子供に道を歩かせる」. This is the favourite N4 trap.
The causative sounds like a superior giving orders, so do not say 「先生に説明させる」.
To ask permission use 「〜させてください」: 「休ませてください」 (Please let me rest).