syanai de naisen wo tukaimasu.

They are read as:

• 社内 → しゃない
• 内線 → ないせん
• 使います → つかいます

So the whole sentence is read:

しゃない で ないせん を つかいます。

社内 literally means inside the company or within the company.

It can refer to:

• the company building or office space
• the company as an internal organization

So it often has the nuance of internally, not just at the company in a broad sense.

で marks the place where an action happens.

In this sentence, the action is 使います (use), and 社内で shows the location or setting where that action takes place.

So:

• 社内で = in the company / within the company
• 内線を使います = use the extension line

A good way to think of で here is: this is where the action happens.

Normally, no.

で is used because 使います is an action, and で marks the location of an action.

By contrast, に is commonly used for things like:

• existence: 会社に人がいます
• destination: 会社に行きます
• specific time: 3時に来ます

So 社内に内線を使います would sound unnatural.

Because 内線 is the direct object of the verb 使います.

The particle を marks the thing that is used.

So:

• 内線を使います = use the extension line
• パソコンを使います = use a computer
• 日本語を使います = use Japanese

This is a very common pattern:

noun + を + 使います

内線 usually means an extension line or internal phone line in a company, school, hospital, and so on.

It refers to a phone system used internally within an organization.

So this sentence is about using the company’s internal phone system, not an outside telephone line.

Yes. 使います is a general verb meaning use, so it is natural if the sentence is talking broadly about using the extension system.

But depending on the situation, Japanese might also use other verbs. For example:

• 内線をかけます = make an internal call
• 内線で話します = talk by extension phone

So:

• 内線を使います focuses on using the extension
• 内線をかけます focuses on placing a call

The subject is omitted, which is very normal in Japanese.

Japanese often leaves out information that is understood from context. So this sentence could mean:

• I use the extension line in the company
• We use the extension line in the company
• Employees use the extension line in the company

The listener is expected to understand the subject from the situation.

Yes. 使います is the polite -ます form.

The plain form is 使う.

So:

• polite: 社内で内線を使います。
• plain: 社内で内線を使う。

The polite form is what you will often see in textbooks and formal everyday speech.

Not completely. Japanese word order is fairly flexible because the particles show each word’s role.

The most neutral order here is:

社内で 内線を 使います。

But 内線を社内で使います。 is also grammatical.

The nuance may shift a little depending on what you want to emphasize, but both are correct. The verb usually stays at the end.

The spaces are there only to help learners read the sentence.

In normal Japanese writing, it would usually be written as:

社内で内線を使います。

Japanese normally does not use spaces between words the way English does.