sensei kara no meeru ga mada midoku no mama datta node, sugu yomimasita.

先生からのメール means an email from the teacher.

• 先生 = teacher
• から = from
• の = links that whole phrase to the noun that follows

So the pattern is:

X からの Y = Y from X

In this sentence:

• 先生からのメール = the email from the teacher

The の turns 先生から into something that can modify メール.

Here, が marks メール as the subject of the descriptive part that follows:

メールがまだ未読のままだった = the email was still left unread

So が is not marking the thing being read yet. It is marking the thing being described.

A rough structure is:

• 先生からのメールが = as for / the email from the teacher
• まだ未読のままだった = was still in an unread state

まだ means still in this sentence.

So:

• まだ未読 = still unread

It shows that, up to that point, the email had not been read yet.

Be careful: まだ can also mean not yet when used with a negative verb, but here it is closer to still because it describes a continuing state.

未読 (みどく) literally means not yet read / unread.

It is especially common in contexts like:

• email
• text messages
• chat apps
• online messages

For example:

• 未読メッセージ = unread messages
• 未読のまま = left unread

It sounds a little formal or tech-related, but it is very natural in modern Japanese.

～のまま means remaining in that state or left as it was.

So:

• 未読のまま = left unread
• そのまま = as it is / just like that

In this sentence:

• 未読のままだった = it had remained unread

This adds the nuance that the email was not just unread, but had been left in that condition until then.

だった is the plain past form of だ.

The sentence is in a mixed style:

• plain form in the reason clause: 未読のままだったので
• polite form in the main action: 読みました

This kind of mix is very common in Japanese.

Why does it happen? Because ので often attaches to plain forms:

• 未読のままだったので = because it had remained unread

If you wanted the whole sentence to be more consistently polite, you could also say:

• 未読のままでしたので、すぐ読みました。

That is also natural.

ので means because / since.

So:

未読のままだったので、すぐ読みました。 = Because it was still unread, I read it right away.

Compared with から, ので usually sounds:

• a bit softer
• a bit more explanatory
• a bit less direct

So:

• AのでB = because A, B
• AからB = because A, B

Both are possible in many cases, but ので often feels more natural when giving a calm reason or explanation.

Because the speaker is describing a completed action:

• the email was unread
• so they read it right away

読みました means read in the sense of I read it / I went ahead and read it.

Japanese often uses past tense for completed actions just like English does.

Yes, すぐ means right away, soon, or immediately, depending on context.

Here:

• すぐ読みました = I read it right away

It shows a quick response after noticing that the email was still unread.

Yes, that would also be natural:

• 先生からのメールをまだ読んでいなかったので、すぐ読みました。
• Because I hadn’t read the teacher’s email yet, I read it right away.

The difference is nuance:

• 未読のままだった focuses on the state of the email: it was left unread.
• 読んでいなかった focuses more directly on the action: I had not read it.

In message/email contexts, 未読のまま sounds especially natural.

A literal breakdown is:

• 先生からのメールが = the email from the teacher
• まだ = still
• 未読のままだった = had remained unread
• ので = because
• すぐ = right away
• 読みました = I read it

So very literally:

Because the email from the teacher was still left unread, I read it right away.

That sounds a little stiff in English, but it matches the Japanese structure closely.