huutou ni zyuusyo wo kaitara, yuubinkyoku de tegami wo dasimasu.

Because 封筒に住所を書く means to write an address on an envelope.

Here, に marks the surface or target location where something is written. In English we say on the envelope, but in Japanese this is often expressed with に.

• 封筒に住所を書く = write the address on the envelope
• 紙に名前を書く = write your name on paper
• 黒板に字を書く = write characters on the blackboard

So に here is not to in the movement sense. It marks the place where the writing appears.

Because 住所 is the direct object of 書く.

The thing being written is the address, so:

• 住所を書く = write the address

In the full phrase:

• 封筒に = on the envelope
• 住所を = the address
• 書いたら = if/when you write it / after writing it

So Japanese separates:

• where you write it: に
• what you write: を

書いたら is the past short form of 書く plus たら.

It often means:

• if you write
• when you write
• after you write

In this sentence, the most natural sense is probably after writing the address on the envelope or once you’ve written the address.

So the sentence describes a sequence:

1. write the address on the envelope
2. mail the letter at the post office

This is a very common use of たら for after doing X, then Y.

No. たら can mean different things depending on context.

Common meanings include:

• if
• when
• after

In this sentence, it does not sound like a pure hypothetical such as if I happen to write it. It sounds more like a step in a process:

• 封筒に住所を書いたら、郵便局で手紙を出します。
• After I write the address on the envelope, I’ll mail the letter at the post office.

So learners should not translate たら as if every time. Context matters.

Because で marks the place where an action happens.

Here, the action is 手紙を出します. That action happens at the post office, so:

• 郵便局で = at the post office

Compare:

• 学校で勉強する = study at school
• レストランで食べる = eat at a restaurant
• 郵便局で手紙を出す = mail a letter at the post office

So で is the normal particle for the location of an action.

Yes, 出す has a broad core meaning of put out / send out / take out, and in the expression 手紙を出す, it means to send/mail a letter.

This is a very common set phrase:

• 手紙を出す = send/mail a letter
• メールを出す can also be used in some contexts for sending mail, though メールを送る is more common for email

So even though the basic verb 出す has many meanings, in this combination it specifically means mail or send off.

Both can relate to sending, but 手紙を出す is a very common, natural expression for mailing a letter.

• 手紙を出す = put a letter into the mail / mail a letter
• 手紙を送る = send a letter

Both can work, but 出す strongly fits the act of posting a letter. Since the sentence also includes 郵便局で, 手紙を出します sounds especially natural.

Because 書いたら is part of a grammatical conditional form, not the main sentence ending.

Japanese politeness is usually shown at the end of the sentence, in the main predicate. Here the main predicate is:

• 出します

That is why it is polite.

The first part:

• 書いたら

is a subordinate clause. It does not need to match the polite style of the final verb.

This is very common:

• 時間があったら、行きます。
• 雨が降ったら、帰ります。

So there is no style mismatch here.

Yes, the subject is omitted, which is very normal in Japanese.

The sentence does not explicitly say I, you, or someone. The subject is understood from context.

Depending on the situation, it could mean:

• After I write the address on the envelope, I’ll mail the letter at the post office.
• After you write the address on the envelope, you mail the letter at the post office.
• Once the address is written on the envelope, the letter gets mailed at the post office.
  (less likely as a direct reading, but possible in some contexts)

Japanese often leaves out information that is already obvious.

You could say 封筒の上に住所を書く, but 封筒に住所を書く is the more natural and standard expression.

Why?

Because with verbs like 書く, Japanese often uses に directly to mark the writing surface:

• ノートに書く
• 紙に書く
• 壁に書く

Using の上に is more literal, like emphasizing the physical top/surface. It is not wrong in all situations, but it is less natural here.

Both can connect two actions in sequence, but they are not exactly the same.

• 書いてから = after writing, and then...
• 書いたら = when/if/after writing...

In this sentence, 書いてから would sound like a straightforward sequence:

• 封筒に住所を書いてから、郵便局で手紙を出します。
• After writing the address on the envelope, I’ll mail the letter at the post office.

With 書いたら, there is a slightly broader nuance. It can still mean sequence, but it leaves room for a when/once feeling. In many everyday contexts, both are possible, but たら is a bit more flexible.

They are:

• 封筒 → ふうとう
• に
• 住所 → じゅうしょ
• を
• 書いたら → かいたら
• 郵便局 → ゆうびんきょく
• で
• 手紙 → てがみ
• を
• 出します → だします

So the full reading is:

• ふうとう に じゅうしょ を かいたら、ゆうびんきょく で てがみ を だします。

The basic meaning can still be understood if some parts move around, because particles show each word’s role. But the given order is very natural.

Standard order here is:

• place/surface + object + verb
• then place of action + object + verb

So:

• 封筒に住所を書いたら、郵便局で手紙を出します。

You might also hear small variations for emphasis, but beginners should stick with this structure.

A good rule is:

• keep each noun with its particle
• put the verb at the end of its clause

That will help your Japanese sound natural.