guruupu ni midoku no messeezi ga takusan tamatte ite, yomuno ni zikan ga kakatta.

Here, グループに means in the group or on the group chat as the place where the messages had accumulated.

With たまる (to accumulate / pile up), the location is often marked by に, because it marks the place where something ends up existing or gathering.

• グループにメッセージがたまる = messages accumulate in the group

If you used で, it would sound more like the group is the place where some action is actively happening. But たまる is not really an action someone performs there; it is a state/result of messages building up. So に fits better.

未読 means unread, but grammatically it behaves like a noun-like word here, so it connects to メッセージ with の.

• 未読のメッセージ = unread messages

This is similar to:

• 日本の会社 = a Japanese company
• 学生の時 = when I was a student

So the の here is not possession. It is just linking 未読 to メッセージ as a modifier.

Because メッセージ is the thing that is doing the accumulating, grammatically speaking.

• 未読のメッセージがたくさんたまっている
• A lot of unread messages have piled up

The verb たまる is intransitive, so it does not take a direct object with を. The messages are the subject, so they are marked with が.

Compare:

• メッセージがたまる = messages accumulate
• メッセージをためる = to accumulate/store messages

So たまる and ためる are an intransitive/transitive pair.

たくさん means a lot / many.

In this sentence, it modifies the idea of how many unread messages there were:

• 未読のメッセージがたくさんたまっていて
• a lot of unread messages had piled up

It is very common to place たくさん before the verb like this.

You could also think of it as describing the amount of messages that accumulated.

たまっていて comes from:

• たまる → たまっている → たまっていて

Breakdown:

1. たまる = to accumulate
2. たまっている = have accumulated / are accumulated / are piled up
3. たまっていて = te-form used to connect to the next clause

So this part means something like:

• messages had piled up, and...
• because messages had piled up...

In this sentence, it smoothly connects the first idea to the second one.

With たまる, 〜ている does not just mean an action is currently happening. It often describes the resulting state.

So:

• たまる = to accumulate
• たまっている = to be accumulated / to have piled up

This is like saying the messages are now in the state of having built up.

This is very common in Japanese with certain intransitive verbs:

• ドアが開いている = the door is open
• 電気がついている = the light is on
• メッセージがたまっている = messages have piled up

It can feel like both, depending on context.

The て-form often links ideas loosely, and English may translate it as:

• and
• so
• because

In this sentence, the meaning is basically:

• unread messages had piled up in the group, and so / because of that it took time to read them

So the connection is not just a simple sequence. It also suggests a reason.

In this sentence, 読むのに is not the same as the more advanced grammar pattern meaning although.

Here, の turns the verb 読む into a noun-like phrase, and に marks the purpose/use/result area:

• 読む = to read
• 読むの = reading / the act of reading
• 読むのに時間がかかった = it took time to read

A very common pattern is:

• Verb dictionary form + のに + time/money/effort + がかかる

Examples:

• 行くのに1時間かかる = it takes one hour to go
• 作るのにお金がかかる = it costs money to make
• 理解するのに時間がかかった = it took time to understand

So here のに means something like for doing or in order to do in a practical sense.

No. They look the same, but they are different grammar patterns.

1. Verb/plain form + のに = although / even though

   • 勉強したのに、忘れた。
   • Even though I studied, I forgot.

2. Verb dictionary form + のに + time/money/etc. がかかる

   • 読むのに時間がかかった。
   • It took time to read.

In your sentence, it is the second pattern.

The object is understood from context: the unread messages.

Japanese often leaves out information that is already obvious. So after saying:

• 未読のメッセージがたくさんたまっていて

it is natural to omit:

• そのメッセージを

and simply say:

• 読むのに時間がかかった

In fuller form, it could be:

• グループに未読のメッセージがたくさんたまっていて、そのメッセージを読むのに時間がかかった。

But the shorter version sounds more natural.

Both are possible in Japanese, but they mean things a little differently.

• 時間がかかった = time was required / it took time
• 時間をかけた = someone spent time on it

Your sentence uses the more neutral expression:

• 読むのに時間がかかった
• It took time to read

This focuses on the fact that the task required time, rather than on the person deliberately investing time.

Compare:

• この本を読むのに時間がかかった。 = It took time to read this book.
• この本を読むのに時間をかけた。 = I spent time reading this book.

Because in Japanese, かかる treats the thing required as the subject.

So:

• 時間がかかる = time is required / it takes time
• お金がかかる = money is required / it costs money

That is why 時間 is marked by が.

This may feel different from English, where we often use a dummy subject like it:

• It took time

But Japanese says it more like:

• Time took/was required

Because the speaker is talking about a completed situation in the past.

• unread messages had piled up
• reading them took time

So かかった is the past form of かかる.

If you changed it to non-past:

• 読むのに時間がかかる

that would mean:

• It takes time to read
• or It will take time to read

depending on context.

Not naturally in this sentence.

• 読むために usually means in order to read
• 読むのに時間がかかった means it took time to read

These are different structures.

For example:

• 本を読むために図書館へ行った。
  • I went to the library in order to read a book.

But:

• 本を読むのに時間がかかった。
  • It took time to read the book.

So のに is the correct pattern with 時間がかかる.

Yes, but it would change the nuance.

The original:

• 未読のメッセージがたくさんたまっていて、読むのに時間がかかった。

This presents the unread messages and the required time as pieces of information in a straightforward way.

If you said:

• 未読のメッセージはたくさんたまっていて...

then 未読のメッセージ becomes more of a topic, as if you are specifically talking about those messages as a known subject.

Similarly, 時間はかかった would add contrast, like:

• It did take time, though...

So が is the more neutral choice here.

It has two linked parts:

1. グループに未読のメッセージがたくさんたまっていて

   • in the group, many unread messages had piled up

2. 読むのに時間がかかった

   • it took time to read them

So the structure is basically:

• [situation/state] + て-form + [result/consequence]

This is a very common Japanese pattern. The first clause sets up the background, and the second clause tells what happened because of that situation.