mousikomisyo ni syomeisite kara, madoguti ni dasimasita.

With 署名する, the document or surface you sign is often marked with に.

• 申込書に署名する = sign on the application form

Here, に marks the target/place where the signature goes.
Using を with 署名する is not the standard pattern here.

So:

• 申込書に署名しました = I signed the application form

not usually:

• 申込書を署名しました

Vてから is a very common grammar pattern meaning after doing V.

So:

• 署名してから = after signing

The sentence shows two actions in order:

1. sign the application form
2. submit it at the counter

So 署名してから、窓口に出しました means the signing happened first, and the submission happened second.

No. Here it means after.

That is because it is attached to the て-form:

• 署名してから = after signing

Compare:

• 署名したから = because I signed
• 署名してから = after signing

So the form before から is very important.

Because these are two different grammar patterns.

• Vてから = after doing V
• Vたから / V plain + から = because

So:

• 署名してから、窓口に出しました = After signing, I submitted it at the counter.
• 署名したから、窓口に出しました would sound like Because I signed it, I submitted it at the counter, which is a different idea and not the natural meaning here.

In Japanese, when actions are linked with Vてから, the tense is usually shown in the final main verb.

So:

• 署名してから、出しました

already tells you that the whole sequence happened in the past.

The first verb appears in て-form because it is connecting to the next action, not because it is present tense.

出す has several meanings, and one very common meaning is to submit or to hand in.

In this sentence:

• 窓口に出しました = submitted it to the counter / handed it in at the counter

So this is not the literal put out meaning. It is the administrative submit meaning, which is very common with forms, documents, homework, applications, and so on.

窓口 literally refers to a service window, but in many situations it means:

• service counter
• front desk
• reception counter
• clerk’s window

So in this sentence, 窓口に出しました means something like I handed it in at the service counter.

It does not have to be a literal glass window.

The two に particles are doing different jobs.

• 申込書に署名する: に marks the place/target where the signature goes
• 窓口に出す: に marks the destination or recipient of the submitted item

So even though both are に, they are not the same kind of に.

It is omitted because it is already obvious from context.

The thing being submitted is the 申込書. A fuller sentence would be:

• 申込書に署名してから、申込書を窓口に出しました。

But Japanese often leaves out repeated words when they are easy to understand.

So the omitted object is basically:

• 申込書を

The subject is omitted because Japanese often leaves out subjects when they are understood from context.

Depending on the situation, it could mean:

• I signed and submitted it
• he/she signed and submitted it
• they signed and submitted it

In many isolated example sentences, English translations often use I, but the Japanese itself does not explicitly say that.

窓口に出しました is the most natural choice here.

• に works well because it marks the target/recipient of the submission.
• へ can sometimes be used for direction, but it is less natural with 出す in this kind of administrative context.
• で marks the place where an action happens, so 窓口で出しました sounds less natural if your main idea is submitted it to the counter.

So for this sentence, 窓口に出しました is the best fit.

Yes, slightly.

• 署名 is more formal and is especially common in official or administrative contexts.
• サイン is also common, but it can sound more casual or less formal depending on the situation.

Since 申込書 is an application form, 署名 is a very natural word here.

It is read:

• もうしこみしょ

It is made up of:

• 申し込み = application
• 書 = document / form

So 申込書 means application form.