wǒmen xūyào diàochá yuányīn.

The sentence follows a very common Mandarin pattern:

subject + 需要 + verb + object

So here it is:

• 我们 = subject
• 需要 = need / need to
• 调查 = investigate
• 原因 = reason / cause

Mandarin often builds sentences in a very straightforward order, so this is structurally similar to We need to investigate the cause/reason.

Because Mandarin does not use an infinitive marker like English to.

In English, you say need to investigate.
In Mandarin, 需要 can be followed directly by a verb:

• 需要调查 = need to investigate
• 需要学习 = need to study
• 需要准备 = need to prepare

So to is simply not something Mandarin needs here.

Yes. 需要 can be followed by:

1. a noun

   • 我们需要时间。
   • We need time.

2. a verb phrase

   • 我们需要调查原因。
   • We need to investigate the reason/cause.

This is very normal Mandarin usage. You do not need to add another helper word between 需要 and 调查.

Because 原因 is the object of the verb 调查.

In Mandarin, the usual order is:

verb + object

So:

• 调查原因 = investigate the reason/cause
• 学习中文 = study Chinese
• 解决问题 = solve the problem

This is one of the most basic word-order patterns in Chinese.

Because 调查原因 is a verb-object phrase, not a possession or modifier phrase.

• 调查原因 = investigate reasons/causes
• 调查的原因 would mean something like the reason that was investigated or the investigated reason, which is a very different structure

So in this sentence, 调查 is the action and 原因 is what is being investigated. No 的 is needed.

It can mean either, depending on context.

Mandarin nouns usually do not change form for singular vs. plural the way English nouns do. So 原因 can be understood as:

• the reason
• the reasons
• the cause
• the causes

Context tells you which one is intended.

Also, Mandarin does not use articles like a, an, and the, so those are also understood from context.

我们 is the normal, neutral word for we/us.

咱们 is more colloquial and usually explicitly includes the listener, like we, including you.

So:

• 我们 = we/us, neutral and widely usable
• 咱们 = we/us including you, often more conversational

In a plain written sentence like this, 我们 is the safest and most standard choice.

It is natural, especially in a more formal, analytical, or professional context.

调查 suggests a real investigation, looking into facts or causes more systematically.

Depending on context, native speakers might also say:

• 查明原因 = find out / clarify the cause
• 了解原因 = understand the reason
• 调查一下原因 = investigate the reason a bit

So 调查原因 is correct, but it can sound somewhat formal or task-oriented.

Yes, but the tone changes.

• 我们要调查原因。 = We are going to investigate the cause / We need to investigate the cause
• 我们需要调查原因。 = We need to investigate the cause

需要 sounds more clearly like need and is slightly more formal or explicit.
要 is very common, but it can mean want, need, or be going to, depending on context.

So 需要 is often clearer for learners.

A natural pronunciation is:

wǒmen xūyào diàochá yuányīn

A few helpful notes:

• 我们 → wǒmen
  • 我 is third tone
  • 们 is usually neutral tone here
• 需要 → xūyào
  • xū = first tone
  • yào = fourth tone
• 调查 → diàochá
  • diào = fourth tone
  • chá = second tone
• 原因 → yuányīn
  • yuán = second tone
  • yīn = first tone

When spoken naturally, the sentence flows smoothly without heavy pauses:

wǒmen xūyào diàochá yuányīn

You can put a little stress on 需要 and 原因 if you want the sentence to sound clear and natural.

Yes, if the subject is already understood from context.

Mandarin often omits subjects when they are obvious. For example, in conversation, someone might simply say:

• 需要调查原因。

This can mean [We] need to investigate the cause, if everyone already knows who is being talked about.

But when you want to be clear, especially in writing or in a stand-alone sentence, keeping 我们 is better.

It is fairly neutral, leaning a little formal because of 需要 and 调查.

It would fit well in contexts like:

• work
• school
• reports
• meetings
• problem-solving discussions

In very casual speech, people might say something slightly softer or more conversational, such as:

• 我们得查一下原因。
• 我们要先看看是什么原因。

But 我们需要调查原因。 is completely correct and natural.