kesa yuubinuke wo mitara, tegami ga haitte imasita.

今朝 is a time expression, and many common time words can appear without a particle.

So:

• 今朝 郵便受けを見たら… = This morning, when I looked in the mailbox…

You could sometimes add は for contrast or emphasis, as in 今朝は, but with a simple neutral statement, leaving it bare is very common and natural.

Other similar examples:

• 今日 行きます = I’m going today.
• 昨日 会った = I met them yesterday.

郵便受け literally means something like mail receiver. In natural English, it can mean:

• mailbox
• letterbox
• sometimes mail slot

The exact image depends on context and country. In this sentence, mailbox or letterbox is the most natural translation.

It is made from:

• 郵便 = mail, post
• 受け = receiver, receptacle

So it is the place where mail gets delivered.

Because 見る normally takes a direct object marked by を.

So grammatically:

• 郵便受けを見た = looked at / checked the mailbox

Even though English often says looked in the mailbox, Japanese commonly uses 見る with the thing being looked at or checked.

In this sentence, 郵便受けを見たら is very natural as when I checked the mailbox.

So the を does not mean the mailbox is being physically affected. It just marks the object of 見る.

たら can mean if or when, depending on context.

Here, it means when:

• 郵便受けを見たら、手紙が入っていました。
• When I looked in / checked the mailbox, there was a letter in it.

Why when and not if? Because this is describing a real past event and what was discovered afterward. The sequence is:

1. I checked the mailbox.
2. I found that a letter was in it.

So 見たら here has the sense of when I looked / when I checked.

A very natural English rendering is:

• When I checked the mailbox this morning, there was a letter in it.

が marks the thing that was newly noticed or identified.

Here, the speaker checked the mailbox and discovered something: a letter.

So:

• 手紙が入っていました = there was a letter in it / a letter was inside

Using が is natural because the letter is being presented as new information.

If you used は, it would sound more like the letter is already the topic and you are saying something about it. That is less natural in this discovery-style sentence.

Also, を would not work with 入る, because 入る is an intransitive verb.

Because the location is already obvious from context.

The first clause says:

• 郵便受けを見たら = when I checked the mailbox

So in the second clause, Japanese can omit the location because it is understood:

• 手紙が入っていました = there was a letter inside

English often needs in it, but Japanese frequently leaves this kind of information unstated when it is clear.

If you wanted to say it more explicitly, you could say:

• 郵便受けに手紙が入っていました
• 郵便受けの中に手紙が入っていました

But the original sentence is perfectly natural.

This is a very important grammar point.

入る means to enter / to go in.

But 入っている often does not mean is entering. With verbs of change like 入る, the 〜ている form often describes the resulting state.

So:

• 入った = went in / entered
• 入っている = is in / has gone in and is there
• 入っていました = was in / had gone in and was there

In this sentence, the speaker is not describing the moment the letter entered the mailbox. They are describing the state they found when they checked it: the letter was inside.

So 入っていました is the correct form.

No. Here it means a state, not an ongoing movement.

With many motion/change verbs, 〜ている can show the result of a completed action.

So:

• ドアが開いている = the door is open
  not usually the door is opening
• 窓が閉まっている = the window is closed
• 手紙が入っている = the letter is in it

In the same way:

• 手紙が入っていました = the letter was in it

So this is best understood as there was a letter inside.

This is a very common learner question.

The いました here is not the existence verb いる being chosen because the letter is animate. It is part of the grammar pattern:

• 入っている

This いる is the auxiliary used in the 〜ている form, and it can be used with inanimate things too.

So:

• 手紙が入っていました is completely normal.

If you said:

• 手紙がありました

that would mean there was a letter, but it would not specifically express the idea that it was inside something in the same way.

So:

• ありました focuses on existence
• 入っていました focuses on the state of being inside

Yes. The subject is omitted, and it is understood from context.

In a sentence like this, the default understanding is that the speaker is the one who looked:

• (私は) 今朝 郵便受けを見たら、手紙が入っていました。

Japanese often leaves out subjects when they are obvious.

So the full sense is something like:

• This morning, when I checked the mailbox, there was a letter in it.

The subject of the first clause is the speaker, while the subject of the second clause is 手紙.

Often, yes.

A very literal translation of 郵便受けを見たら is when I looked at the mailbox, but in natural English that can sound slightly odd, because what the speaker really did was inspect it to see whether mail had arrived.

So in context, a more natural translation is:

• When I checked the mailbox this morning, there was a letter in it.

That captures the intended meaning better, even though the Japanese verb is still 見る.