lùyóuqì zài diànnǎo hòumiàn.

Here, 在 means to be at / to be located at.

So 路由器在电脑后面 means literally:

• 路由器 = router
• 在 = is located at
• 电脑后面 = behind the computer

In sentences about location, 在 often works like English is/are ... at/in/on/behind.

Mandarin usually puts location information in this pattern:

thing + 在 + place + location word

So:

• 路由器 = the router
• 在 = is located
• 电脑后面 = behind the computer

That gives:

路由器 在 电脑 后面。

English says The router is behind the computer, but Mandarin builds it more like:

The router is located at the computer’s back side.

This word order is very common.

Because 是 is generally not used to say where something is located.

For location, Mandarin normally uses 在, not 是.

• 路由器在电脑后面。 = The router is behind the computer. ✅
• 路由器是电脑后面。 = incorrect in normal Mandarin ❌

A simple rule:

• 是 = to identify something
  
  • 这是路由器。 = This is a router.
• 在 = to say where something is
  
  • 路由器在电脑后面。 = The router is behind the computer.

后面 is a common location word meaning behind / the back.

In modern Mandarin, position words are often formed like this:

• 上面 = on top / above
• 下面 = under / below
• 前面 = in front
• 后面 = behind
• 里面 = inside
• 外面 = outside

Using just 后 by itself is usually not the most natural choice here. 后面 sounds much more normal in everyday speech.

电脑后面 means behind the computer or literally the computer’s back side.

It is made of:

• 电脑 = computer
• 后面 = back / behind

Together, it means the space or position behind the computer.

Both 电脑后面 and 电脑的后面 can be correct, but 电脑后面 is very common and natural.

With location words like 前面 / 后面 / 上面 / 里面, Mandarin often omits 的 after a noun:

• 桌子上面 = on the table
• 房子后面 = behind the house
• 电脑后面 = behind the computer

Adding 的 is possible:

• 电脑的后面

But in many simple location phrases, it sounds smoother without 的.

They are very similar. Both can mean behind or the back side.

So these are both natural:

• 路由器在电脑后面。
• 路由器在电脑后边。

A small difference:

• 后面 is very common in both speech and writing.
• 后边 is also common, often a bit more colloquial in some speakers’ usage.

For a learner, 后面 is a very safe choice.

No, not in a normal sentence.

You need 在 to connect the thing to its location:

• 路由器在电脑后面。 ✅
• 路由器电脑后面。 ❌

Without 在, the sentence feels incomplete.

Mostly yes. In this sentence, the most natural meaning is a physical location: the router is physically behind the computer.

后面 can also be used more abstractly in other contexts, such as later or afterwards, but not here.

For example:

• 后面还有内容。 = There is more content later/on the following pages.

But in 路由器在电脑后面, it clearly means physical space.

It is pronounced lùyóuqì.

Tone by tone:

• 路 = lù (4th tone)
• 由 = yóu (2nd tone)
• 器 = qì (4th tone)

So the full pattern is:

4th tone + 2nd tone + 4th tone

If helpful, you can say it slowly at first:

lù - yóu - qì

A very useful pattern is:

A 在 B + 方位词

where:

• A = the thing whose location you are describing
• B = the reference object
• 方位词 = location word, such as 上面, 下面, 前面, 后面, 里面

Examples:

• 书在桌子上面。 = The book is on the table.
• 猫在椅子下面。 = The cat is under the chair.
• 车在房子前面。 = The car is in front of the house.
• 路由器在电脑后面。 = The router is behind the computer.

This is one of the most important basic sentence patterns in Mandarin.

Yes. That is also natural, but the focus changes.

• 路由器在电脑后面。
  Focus: Where is the router?
  Answer: It is behind the computer.

• 电脑后面有路由器。
  Focus: What is behind the computer?
  Answer: There is a router there.

So both can describe the same situation, but they answer different kinds of questions.

电脑 means computer in general.

Depending on context, it could refer to:

• a desktop computer
• a laptop
• a computer system more generally

So in this sentence, you would usually understand it simply as the computer. The exact type depends on the situation.

No. You do not need a measure word here.

Measure words are typically needed when a noun is counted or modified by a number or demonstrative, such as:

• 一个路由器 = one router
• 那台电脑 = that computer

But in 路由器在电脑后面, the nouns are being used directly, so no measure word is needed.

Yes, depending on context, English might translate it either way.

后面 literally points to the back side / rear area of something, so English could be:

• behind the computer
• at the back of the computer

In most everyday situations, behind the computer is the most natural translation.