watasi ha kyouzyuu ni mousikomisyo wo dasimasu.

A natural reading is:

わたし は きょうじゅう に もうしこみしょ を だします。

Word readings:

• 私 = わたし
• 今日中 = きょうじゅう
• 申込書 = もうしこみしょ
• 出します = だします

In normal Japanese writing, this sentence would usually be written without spaces: 私は今日中に申込書を出します。

今日中に means by the end of today or sometime before today is over.

It does not just mean today in a general sense. It gives a deadline.

Compare:

• 今日出します。 = I will submit it today.
• 今日中に出します。 = I will submit it by the end of today.

So 今日中に emphasizes completion before the day ends.

Here, に marks the time limit / deadline point for the action.

So:

• 今日中に = by the end of today

In this sentence, に connects the time expression to the verb and shows when the action must be completed by.

This is a common pattern:

• 明日までに = by tomorrow
• 三時までに = by 3 o’clock
• 今日中に = by the end of today

は marks 私 as the topic of the sentence.

So 私は means something like:

• As for me, ...
• I, ...

It does not always translate directly into English, but it sets up what the sentence is about.

Here, 私は tells us that the speaker is talking about their own action:

• 私は今日中に申込書を出します。

In many situations, 私 could be omitted if it is already obvious who is speaking.

Both can be possible in Japanese, but they have different nuances.

• 私は presents I as the topic.
• 私が would put stronger focus on I as the one who will do it.

So:

• 私は今日中に申込書を出します。 = As for me, I will submit the application form by the end of today.
• 私が今日中に申込書を出します。 = I will submit the application form by the end of today. (perhaps not someone else)

In a neutral statement like this, は is the more natural default.

を marks the direct object of the verb.

Here, the thing being submitted is 申込書:

• 申込書を出します = submit the application form

So を shows what the action is being done to.

This is very common with action verbs:

• 本を読みます = read a book
• 水を飲みます = drink water
• 申込書を出します = submit an application form

Yes, 出す has a broad basic meaning like put out, take out, or send out, but in many contexts it also means submit, hand in, or turn in.

With documents, forms, homework, reports, and applications, 出す is very commonly used this way.

Examples:

• 宿題を出す = hand in homework
• レポートを出す = submit a report
• 申込書を出す = submit an application form

So here 出します is best understood as will submit.

出します is the polite non-past form.

That means it can mean:

• I will submit
• sometimes I submit, depending on context

In this sentence, it most naturally means I will submit.

Politeness levels:

• 出します = polite
• 出す = plain / dictionary form

So this sentence is appropriate in everyday polite conversation, at work, or when speaking respectfully.

Japanese normally follows an SOV pattern:

• Subject / Topic + Object + Verb

So the verb usually comes at the end.

In this sentence:

• 私は = topic
• 今日中に = time deadline
• 申込書を = object
• 出します = verb

That final verb is very natural in Japanese sentence structure.

It can often be omitted.

Japanese frequently leaves out subjects when they are clear from context. So if it is already obvious that you are the one speaking, you could simply say:

今日中に申込書を出します。

This still naturally means I will submit the application form by the end of today.

Adding 私は can make the sentence clearer, more contrastive, or more complete depending on context.

Sometimes yes, but the nuance is a little different.

• 今日中に = by the end of today / before today ends
• 今日までに = by today / no later than today

In many situations they are very close. However, 今日中に strongly focuses on the inside of today as the time window, while 今日までに emphasizes today as the deadline limit.

For this sentence, 今日中に sounds very natural if you mean I’ll get it done before the day is over.

Usually, yes. In this kind of sentence, 出します sounds like a fairly direct statement of intention or plan.

It can sound like:

• a decision
• a promise
• a firm plan

So 私は今日中に申込書を出します。 sounds stronger than a vague idea like I might submit it today.

If you wanted to sound less definite, you might use other expressions instead. But as written, this sounds like a clear intention.