chātóu zài guāngmāo hòumiàn.

Mandarin often uses this basic pattern for location:

thing + 在 + place + location word

So here:

• 插头 = the plug
• 在 = is located at / in / on / behind, depending on the location phrase after it
• 光猫 = the optical modem / fiber modem
• 后面 = back / behind

So the sentence literally works like:

Plug + is located at + optical modem + back

Natural English becomes The plug is behind the optical modem.

A very common Mandarin pattern is:

• 书在桌子上面。 = The book is on the table.
• 猫在椅子下面。 = The cat is under the chair.
• 插头在光猫后面。 = The plug is behind the optical modem.

Here, 在 is a location verb / preposition-like word meaning to be at / to be located at.

In this sentence, it tells you where something is:

• 插头在光猫后面。 = The plug is behind the optical modem.

So 在 is not just a general word for at. In many sentences, it functions like is located at/in/on.

Compare:

• 我在家。 = I am at home.
• 手机在桌子上。 = The phone is on the table.
• 插头在光猫后面。 = The plug is behind the optical modem.

Because 是 is usually used to connect A = B:

• 我是学生。 = I am a student.
• 这是光猫。 = This is an optical modem.

But in this sentence, you are not identifying the plug as the back of the modem. You are describing where the plug is located.

For location, Mandarin normally uses 在, not 是:

• 正确: 插头在光猫后面。
• 不自然 / incorrect here: 插头是光猫后面。

So:

• 是 = equals / is
• 在 = is at / is located at

后面 means behind or the back side.

It is made up of:

• 后 = back / rear / behind
• 面 = side / surface

Together, 后面 literally suggests the back side.

This kind of word is very common in Mandarin. Other similar location words include:

• 前面 = in front
• 上面 = above / on top
• 下面 = below / underneath
• 里面 = inside
• 外面 = outside
• 旁边 = beside

These words often come after a noun to describe location:

• 桌子后面 = behind the table
• 光猫后面 = behind the optical modem

Yes. Both are natural:

• 插头在光猫后面。
• 插头在光猫的后面。

The version without 的 is often very common and concise in everyday speech.

The version with 的 can sound a little more explicit or slightly more formal in some contexts, but both are correct.

This is common with location phrases:

• 桌子上面 / 桌子的上面
• 沙发后面 / 沙发的后面

In many everyday sentences, learners will hear the no 的 version very often.

They are very close in meaning. Both can mean behind / the back.

• 后面
• 后边

In many situations, they are interchangeable:

• 插头在光猫后面。
• 插头在光猫后边。

Both are understandable and natural.

Very broadly:

• 后面 may sound a bit more standard or neutral
• 后边 may sound a bit more colloquial in some speakers’ usage

But this is not a major difference for a learner at this level.

光猫 is a common colloquial Chinese term for a fiber-optic modem or optical network terminal.

It is short for something related to 光纤猫 or optical modem usage. The 猫 here comes from modem being informally rendered as 猫 in Chinese tech slang.

So although 猫 normally means cat, in this word it has nothing to do with an animal.

Learners are often surprised by this, but it is a real and common tech word in everyday Chinese.

插头 usually means plug: the part that gets inserted into a socket or port.

Depending on context, it can refer to:

• a power plug
• a cable plug
• another kind of connector end

If the situation is about home electronics, many people will probably understand it as a power plug unless context suggests otherwise.

Related words:

• 插座 = socket / outlet
• 电源插头 = power plug
• 网线插头 = network cable plug / connector

So in this sentence, 插头 is a general word for the plug.

Mandarin does not have articles like a, an, and the in the same way English does.

So:

• 插头 can mean a plug, the plug, or just plug
• 光猫 can mean an optical modem, the optical modem, or just optical modem

The exact meaning depends on context.

In real life, if both speaker and listener already know which plug and which modem are being discussed, English naturally uses the:

• The plug is behind the optical modem.

Mandarin simply leaves that to context.

Yes, it is a completely normal and complete sentence.

Mandarin often allows very compact location statements when the context is clear.

This sentence has everything needed:

• subject: 插头
• location verb: 在
• location phrase: 光猫后面

So it is fully grammatical.

Short, practical sentences like this are very common in spoken Chinese, especially when giving directions or describing where something is.

Yes, and the meaning changes slightly in focus.

1. 插头在光猫后面。
   Focus: the plug and where it is
   = The plug is behind the optical modem.

2. 光猫后面有插头。
   Focus: what exists behind the optical modem
   = There is a plug behind the optical modem.

So:

• A 在 B 后面 = A is behind B
• B 后面有 A = There is an A behind B

Both are natural, but the topic/focus is different.

A common way is to add 吗:

• 插头在光猫后面吗？
• Is the plug behind the optical modem?

You can also use an A-not-A question pattern:

• 插头在不在光猫后面？
• Is the plug behind the optical modem or not?

Both are common.

You negate 在 with 不:

• 插头不在光猫后面。

This is the normal pattern:

• 在 → 不在

Examples:

• 我在家。 = I’m at home.
• 我不在家。 = I’m not at home.
• 手机在桌子上。 = The phone is on the table.
• 手机不在桌子上。 = The phone is not on the table.

A few parts may be worth extra attention:

• 插头 = chātóu

  • 插 is first tone: chā
  • 头 here is second tone: tóu

• 在 = zài

  • fourth tone

• 光猫 = guāngmāo

  • 光 first tone
  • 猫 first tone

• 后面 = hòumiàn

  • 后 fourth tone
  • 面 fourth tone

A common learner issue is making 后面 too flat. Both syllables should be clearly fourth tone in careful speech: hòu-miàn.

Yes, if the context already makes it clear behind what you mean.

• 插头在后面。 = The plug is in the back / behind it / behind there.

But by itself, it is less specific. Usually, if you want to be clear, you include the reference object:

• 插头在光猫后面。

So omitting 光猫 is possible only when the listener already knows the reference point.