kyou nyuusyasimasita.

Here, 今日 is read きょう, meaning today.

A native English speaker may notice that 今日 can sometimes be read differently, especially in set expressions, but in an everyday sentence like this, きょう is the normal reading.

入社しました is the polite past form of 入社する.

• 入社 = joining a company
• する = to do
• 入社する = to join a company
• 入社しました = joined a company / entered a company

So this is a noun + する verb pattern, which is very common in Japanese.

Examples of similar patterns:

• 勉強する = to study
• 電話する = to phone
• 入院する = to be hospitalized

Because time expressions like 今日, 昨日, 明日, 先週, and 来年 often appear without a particle.

So:

• 今日入社しました。 = natural
• 昨日行きました。 = natural
• 明日帰ります。 = natural

You will sometimes see に with certain time words, but with words like 今日, 昨日, and 明日, it is usually not used.

So 今日に入社しました would sound unnatural.

Japanese uses the past form here because the action is viewed as completed.

So 今日入社しました means that the speaker joined the company today, and that joining has already happened.

This is similar to English:

• I joined the company today.

Even though today includes the present day, the act of joining is treated as a completed event, so the past tense is natural.

Because Japanese often leaves out information that is clear from context.

So 今日入社しました。 naturally implies something like:

• I joined the company today.

But Japanese does not need to say 私は unless there is a reason to emphasize or contrast the subject.

For example:

• 私は今日入社しました。 = I joined the company today.

This is grammatical, but in many real situations, 私は is unnecessary.

Both can relate to joining a company, but they are not exactly the same.

• 入社しました = joined a company / entered employment at a company
• 会社に入りました = entered the company / joined the company

入社する is a more specific and standard word for joining a company as an employee. It is common in business settings.

会社に入る is more general. It can also mean joining a company, but 入社する sounds more formal and more precise in workplace contexts.

So in a business situation, 入社しました is often the more natural choice.

入社 means joining the company as an employee.

It does not usually mean simply physically entering the office building. For that, Japanese would use a different expression, such as one meaning go into the building or enter the office.

Depending on context, 入社しました can suggest:

• I officially joined the company today.
• Today was my first day as a company employee.

So it is about employment status, not just movement into a place.

Because the verb ends in ました, which is the polite past ending.

• 入社した。 = plain past
• 入社しました。 = polite past

The polite form is common when:

• speaking to someone you do not know well
• speaking in business situations
• making a formal statement

Since 入社 is already a business-related word, the polite form fits very naturally here.

Yes, absolutely.

It would sound natural in a business or workplace context, especially if someone is introducing themselves as a new employee.

For example:

• 今日入社しました山田です。よろしくお願いします。
  = I’m Yamada. I joined the company today. Nice to meet you.

Or more naturally with a pause:

• 今日入社しました、山田です。よろしくお願いします。

So the sentence works well as part of a formal introduction.

The most natural order is:

• 今日 入社しました。

Japanese word order is more flexible than English, but time expressions like 今日 often come near the beginning.

You could also say:

• 入社しました、今日。

but that would sound unusual in normal speech unless you are adding emphasis or speaking dramatically.

So for a learner, 今日入社しました。 is the safest and most natural pattern.

The kanji are:

• 入 = enter
• 社 = company / corporation

So 入社 literally gives the idea of entering a company.

This can help you remember the word.

Be careful, though: literal meanings are useful for memory, but the actual word means to join a company as an employee, not just to walk into a company building.

You would normally add the company before 入社しました.

For example:

• 今日、ABC会社に入社しました。
  = I joined ABC Company today.

Here, に marks the company you joined.

So:

• 今日入社しました。 = I joined a company today.
• 今日ABC会社に入社しました。 = I joined ABC Company today.

Without a company name, the sentence still sounds natural if the company is already understood from context.

In speech, there often is a slight pause:

• 今日、入社しました。

In writing, a comma is optional. Japanese often omits commas in short simple sentences, so both are fine:

• 今日入社しました。
• 今日、入社しました。

The comma can make the sentence slightly easier to read, but it is not required.