JLPT N4 Grammar

With transitive verbs

With transitive verbs (those that can be used with a grammatical object, marked with the particle を), the person who does the action, actually the person who is made to do the action, is always marked with the particle に.

In short, sentences of the type Ⓐ will become sentences of the type Ⓑ at the causative form:
Ⓐ [PERSON] が [OBJECT] を [VERB]する
↓
Ⓑ [PERSON 1] が [PERSON 2] に [OBJECT] を [VERB]させる

For example:
Ⓐ 生せい徒と が 作さく文ぶん を 書かく。 ("The students write an essay.")
↓
Ⓑ 先せん生せい が 生せい徒と に 作さく文ぶん を 書かかせる。 ("The teacher makes the students write an essay.")

With intransitive verbs

With intransitive verbs (those that cannot be used with a grammatical object), the person who does the action, actually the person who is made to do the action, can be marked with the particles に OR を, depending on a few factors.

Cases where を is the only option

✔ When we are not talking about a person. For example:
Ⓐ トマト が　腐くさってしまった。 ("The tomatoes rotted.")
↓
Ⓑ トマト を　腐くさらせてしまった。　 ("I made/let the tomatoes rot.")

✔ When we cause an emotion. For example:
Ⓐ 彼かの女じょ が　笑わらう／怒おこる／泣なく。 ("She laughs/gets angry/cries.")
↓
Ⓑ 彼かの女じょ を　笑わらわせる／怒おこらせる／泣なかせる。 ("I make her laugh/angry/cry.")

Cases where に is the only option

✔ When the verb is used with the particle of motion を. For example:
Ⓐ 子こ供ども が 山やま道みち を 歩あるく。 ("The children walk along the mountain path.")
↓
Ⓑ 子こ供ども に 山やま道みち を 歩あるかせる。 ("I make the children walk along the mountain path.")
Be careful, を here does not mark the grammatical object but a place where something/someone is moving. For example:
空そら を 飛とぶ。 ("Fly through the sky.")
家いえ の 前まえ を 通とおる。 ("To pass in front of the house.")

Cases where both に & を can be used

✔ In other cases, both に & を can be used. For example:
Ⓐ 子こ供ども が 泳およぐ。 ("The children swim.")
↓
Ⓑ 子こ供ども を／に 泳およがせる。 ("I make/let the children swim.")

Still alive? Do you want to know more about the causative form? Check out how it is used to ask for permission!