mousikomisyo ni zyuusyo to denwabangou wo kaitara, madoguti ni dasite kudasai.

Because 書く often uses two different things:

• the content you write → marked by を
• the place/surface/document you write it on → often marked by に

So in:

• 申込書に = on the application form
• 住所と電話番号を = the address and phone number

You are writing the address and phone number on the application form.

A very similar pattern is:

• 紙に名前を書く = write your name on paper

Because 住所と電話番号 is one combined object.

Here と connects the two nouns into a complete list:

• 住所 = address
• 電話番号 = phone number
• 住所と電話番号 = address and phone number

Then the whole list is marked by を:

• 住所と電話番号を = the address and phone number

This is the same as English using one object for two linked items.

書いたら is the -たら form of 書く.

• 書いた = wrote / have written
• 書いたら = if you write / when you write / after you write

In this sentence, it does not really mean a doubtful if. In instructions, -たら often means:

• when you’ve done X
• after doing X
• once X is finished

So here it means something like:

• After you fill in the address and phone number, please submit it at the counter.

It shows the order:

1. write it
2. then submit it

Both can work for a sentence like this, and both can mean after writing.

But there is a slight nuance:

• 書いてから strongly emphasizes do A, and then do B
• 書いたら often sounds like once A is done / when you’ve done A, do B

In practical instructions, -たら is very common and natural.

So:

• 書いてから、出してください = write it first, then submit it
• 書いたら、出してください = once you’ve written it, please submit it

The difference is small here.

Here に marks the destination/target of 出す.

• 窓口 = service window / counter / office desk
• 窓口に出す = submit it to the counter

So this に is different from the earlier に after 申込書.

There are two different roles:

• 申込書に = on the application form
• 窓口に = to the counter

Japanese often uses the same particle for different but related functions, so you have to understand it from the verb and context.

Yes, 出す has a broad meaning: put out, take out, send out, submit, and more.

In office or form-filling situations, 出す often means:

• hand in
• submit
• turn in

So 窓口に出してください means:

• please submit it at the counter
• please hand it in at the counter

Also, 出して is the て-form of 出す, and ください makes it a polite request:

• 出してください = please submit it

Japanese often omits the subject when it is obvious.

In an instruction like this, the understood subject is you:

• (あなたは) 申込書に住所と電話番号を書いたら、窓口に出してください。

But saying あなたは here would usually sound unnecessary or unnatural unless there were special emphasis.

This is very normal in Japanese:

• if context makes the subject obvious, it is often left out

Because only the main request needs to carry the politeness here.

The sentence has two parts:

1. 書いたら = subordinate clause
2. 出してください = main clause

In Japanese, subordinate clauses often use the plain form even in polite speech. The politeness is shown in the main clause:

• 出してください = polite request

So it is completely natural to mix:

• plain form in the condition/time clause
• polite form in the main sentence

This is a very common pattern.

Yes. Even though 申込書 is marked by に in the first part, it is still understood to be the thing you later submit.

The sentence structure is basically:

• On the application form, write the address and phone number
• then submit it to the counter

Japanese often avoids repeating the object if it is obvious. So after 書いたら, the omitted object of 出してください is understood to be the application form.

In English we might repeat it, but in Japanese it is often left unstated.

Here are the common readings:

• 申込書 → もうしこみしょ
• 住所 → じゅうしょ
• 電話番号 → でんわばんごう
• 書いたら → かいたら
• 窓口 → まどぐち
• 出して → だして

A full reading of the sentence is:

もうしこみしょ に じゅうしょ と でんわばんごう を かいたら、 まどぐち に だして ください。

Normally, に is the natural choice here because it marks the target/destination of submission.

• 窓口に出す = submit it to the counter

Using で would not sound right in this pattern, because で usually marks:

• the place where an action happens
• the means/tool used

Here the counter is not just the location of the action; it is the place/person-side to which the form is handed. That is why に fits better.