tomodati ga rusu datta node, rusuden ni messeezi wo iremasita.

No. In normal Japanese, this would usually be written without spaces:

友達が留守だったので、留守電にメッセージを入れました。

Textbooks and learning materials often add spaces to help beginners see the parts more clearly, but native Japanese writing normally does not separate words this way.

留守（るす） means that someone is not home, away, or not present.

In this sentence, 友達が留守だった means the friend was absent / not there.

A useful thing to know is that 留守 is a noun, but in Japanese it can be used with the copula to describe a state:

• 留守だ = is away / is not home
• 留守だった = was away / was not home

So even though English might use an adjective-like expression, Japanese is using a noun + copula pattern.

留守だった is the plain past form of 留守だ.

• 留守だ = is away
• 留守だった = was away

Because it is followed by ので, the sentence needs the plain form before ので:

• 留守だったので = because the friend was away

You could also say 留守でしたので, which is more polite and a bit more formal, but 留守だったので is very natural in everyday speech, especially when the final verb is polite:

• 留守だったので、メッセージを入れました。

This mix of plain + polite is extremely common in Japanese.

Both ので and から can mean because, but they have slightly different tones.

• ので sounds a little softer, more explanatory, and often more neutral
• から can sound a little more direct or subjective

So:

• 友達が留守だったので… = Because my friend was away...
• 友達が留守だったから… = Because my friend was away...

Both are correct here, but ので often feels a bit smoother in this kind of explanation.

A rough way to think about it:

• ので: presenting a reason
• から: asserting a reason

In 友達が留守だった, が marks 友達 as the thing being described.

The sentence is focusing on the fact that the friend was away.

So the structure is roughly:

• 友達が = as for who was away? the friend
• 留守だった = was away

Using が here is very natural when introducing or identifying the subject of the situation.

If you said 友達は留守だったので, that would also be possible, but it gives a slightly different nuance:

• が: focuses more on the friend as the subject of this situation
• は: presents the friend as the topic, perhaps with a contrastive feel depending on context

In many everyday situations, both are possible, but が fits very naturally here.

留守電（るすでん） is a shortened casual word for 留守番電話（るすばんでんわ）, which means answering machine or voicemail.

So:

• 留守番電話 = full form
• 留守電 = shortened form, very common in speech and casual writing

Japanese shortens words like this very often.

Here, に marks the target or destination of the action.

• 留守電にメッセージを入れる = to leave/put a message on the answering machine / in voicemail

You can think of に here as meaning something like:

• onto
• in
• to

not as a direct English translation, but as the place/target where the message is being left.

So:

• 留守電に = on the voicemail / into the answering machine system

を marks the direct object of the verb.

The verb is 入れました from 入れる, so the thing being put in / left is メッセージ.

• メッセージを入れました = left a message

So the pattern is:

• 留守電に = onto the voicemail
• メッセージを = the message
• 入れました = put in / left

Yes, the basic meaning of 入れる（いれる） is to put in, insert, or place into.

But in Japanese, 留守電にメッセージを入れる is a very natural expression meaning to leave a message.

So this is an example where Japanese uses a literal put in expression where English usually says leave.

Other examples of 入れる:

• かばんに本を入れる = put a book in a bag
• コーヒーに砂糖を入れる = put sugar in coffee

And in this sentence:

• 留守電にメッセージを入れる = leave a message on voicemail

Because the action of leaving the message is already completed.

The sequence is:

1. The friend was away
2. So a message was left

That is why the sentence uses:

• 留守だった = was away
• 入れました = left / put in

Both are past because the speaker is talking about something that already happened.

Japanese often omits subjects when they are clear from context.

So in this sentence, the person who left the message is not explicitly stated, but it is naturally understood to be the speaker:

• (私は) 友達が留守だったので、留守電にメッセージを入れました。

The 私は is omitted because Japanese usually leaves out information that does not need to be said.

This is one of the most common features of Japanese, and it can feel strange to English speakers at first because English usually requires an explicit subject.

It is a mix of plain and polite forms, which is very common in Japanese.

• 留守だったので = plain-style part
• 入れました = polite past verb

This kind of sentence is completely natural. Japanese often keeps the final verb polite while earlier parts of the sentence stay in plain form.

If you wanted to make it sound more fully polite, you could say:

友達が留守でしたので、留守電にメッセージを入れました。

But the original sentence is already natural and polite enough for many situations.