kanozyo ha rusuden ni namae wo irete oita node, sensei ha sugu orikaesite kuremasita.

Because 留守電に名前を入れる treats 留守電 as the place/device/target into which something is put.

• 名前を入れる = to put in / leave one’s name
• 留守電に = into the voicemail / on the answering machine

So に marks where the name is being left. Using を with 留守電 here would sound unnatural.

Literally, yes, it is put in one’s name, but in this context it means something like:

• leave one’s name
• give one’s name in a voicemail message
• identify oneself on the answering machine

Japanese often uses 入れる in situations where English would use a more specific verb like leave, enter, or record.

～ておく often means:

• do something in advance
• do something and leave it that way
• do something for future convenience/preparation

So:

• 入れた = she left her name
• 入れておいた = she had left her name in advance / she made sure to leave her name

Here, 入れておいた suggests a purposeful action that helped lead to what happened next.

Both ので and から can mean because, but the nuance is a little different.

• ので sounds a bit more explanatory, softer, and more objective
• から can sound more direct or more like the speaker’s own assertion

In this sentence, ので fits well because it calmly explains the reason for the teacher’s response.

折り返す is often used for returning a call.

So すぐ折り返してくれました means the teacher called back right away.

This is a very common phone-related expression in Japanese.
For example:

• 後で折り返します = I’ll call back later
• 折り返しご連絡します = I will contact you back

～てくれる means someone does something for the speaker or in a way the speaker sees as beneficial.

So:

• 折り返した = called back
• 折り返してくれた = called back for me / kindly called back

Using くれました adds a sense that the speaker appreciates the teacher’s action or sees it as helpful.

Using は presents 先生 as the topic for the second part of the sentence:

• As for the teacher, he/she called back right away

If it were 先生が, the focus would be more specifically on the teacher as the one who did the action.

In this sentence, は sounds natural because the speaker is simply continuing the story and stating what the teacher did.

It depends on context. 彼女 can mean:

• she
• girlfriend

In a sentence like this, it is usually just she, unless the surrounding context clearly shows it means girlfriend.

Also, は marks her as the topic of the first action: she is the person who left her name.

Because the sentence describes two completed actions in sequence:

1. She had left her name on the voicemail.
2. As a result, the teacher called back right away.

Both happened in the past, so both verbs appear in past form.

Also, 入れておいたので shows the earlier action as the reason for the later one.

すぐ means right away or immediately, and here it modifies 折り返してくれました.

So the sense is:

• The teacher called back immediately

Its position before the verb phrase is very natural in Japanese.

Yes. 留守電 is a shortened, casual form of 留守番電話.

Both refer to:

• an answering machine
• voicemail

In everyday speech, 留守電 is very common.

Not exactly, unless the original context specifically says she left a full message.

The Japanese here says 名前を入れておいた, which more specifically means she left her name or identified herself.

A full message would more naturally be expressed with something like:

• メッセージを入れる = leave a message

So 名前を入れる is narrower than leave a message.