zhè shì zhēnzhèng de yuányīn.

Because 这 is standing alone as a pronoun, meaning this.

• 这是真正的原因。 = This is the real reason.
• Here, 这 is the subject of the sentence.

You use 这个 when this directly modifies a noun:

• 这个原因 = this reason
• 这个问题 = this problem

So:

• 这是真正的原因。 = This is the real reason.
• 这个是真正的原因。 is possible in some contexts, but it usually sounds like This one is the real reason and can feel more contrastive.

It is similar, but not exactly the same.

In this sentence, 是 links the subject and the noun phrase:

• 这 = this
• 是 = is / am / are (roughly)
• 真正的原因 = the real reason

So yes, here 是 works a lot like is in English.

But Mandarin 是 is not used in every place where English uses is. For example:

• 他是老师。 = He is a teacher.
• But: 他很高。 = He is tall.
  Not 他是高.

So 是 is mainly used when connecting one noun/pronoun to another noun phrase, not usually before adjectives.

真正 means real, genuine, or true in the fullest sense. It often sounds a little more formal or emphatic than 真的.

In this sentence:

• 真正的原因 = the real reason / the actual reason

Compare:

• 真正的朋友 = a true/genuine friend
• 真正的问题 = the real problem

真的 can also mean real or true, but very often it means really? or really in conversation:

• 真的吗？ = Really?
• 这是真的。 = This is true.

So in a phrase like the real reason, 真正的原因 is very natural.

的 is used here to connect a modifier to a noun.

• 真正 modifies 原因
• So: 真正的原因 = the real reason

This is one of the most common uses of 的 in Mandarin.

Structure:

• modifier + 的
  • noun

Examples:

• 重要的问题 = important question
• 漂亮的房子 = beautiful house
• 真正的原因 = real reason

Without 的, the phrase would sound unnatural here for most learners and in normal speech.

Usually, no. The natural form is 真正的原因.

For learners, the safe rule is:

• with a modifier like 真正, use 的 before the noun

So say:

• 这是真正的原因。

Not:

• 这是真正原因。 ✘

In some very formal or literary styles, Chinese can sometimes omit 的 in places where you might expect it, but that is not the normal pattern you should copy here.

The structure is:

• 这 = subject
• 是 = linking verb
• 真正的原因 = predicate noun phrase

So the pattern is:

• A + 是 + B
• This + is + the real reason

Inside 真正的原因, you have:

• 真正 = modifier
• 的 = linking particle
• 原因 = noun

So the full breakdown is:

• 这 / 是 / 真正的原因
• This / is / the real reason

原因 means cause or reason, especially the underlying cause of something.

• 失败的原因 = the reason/cause for failure
• 真正的原因 = the real reason

理由 also means reason, but it is often closer to grounds, justification, or an explanation someone gives.

Compare:

• 迟到的原因 = the cause/reason for being late
• 你没有理由生气。 = You have no reason/justification to be angry.

So in 这是真正的原因, 原因 is the best choice because it refers to the actual cause.

A natural pronunciation is:

• 这 zhè — 4th tone
• 是 shì — 4th tone
• 真正 zhēnzhèng — 1st tone + 4th tone
• 的 de — neutral tone
• 原因 yuányīn — 2nd tone + 1st tone

So altogether:

• zhè shì zhēnzhèng de yuányīn

A few tips:

• 这 and 是 are both 4th tone, so each one falls sharply.
• 的 is light and unstressed.
• Stress often falls more on 真正 and 原因, especially if you are emphasizing that this is the real reason.

Yes, depending on context, 原因 can be translated as reason or cause.

• If the situation is more about explanation, reason may sound better.
• If it is more about what actually produced an effect, cause may sound better.

So 这是真正的原因。 could be:

• This is the real reason.
• This is the real cause.
• This is the actual reason.

The exact English wording depends on context.

Not usually. 这是真正的原因。 means This is the real reason, where 这 means this by itself.

If you want to say This reason is the real one, you would normally make this reason into a full noun phrase:

• 这个原因才是真正的原因。
• 这个原因才是真的。 depending on context

So the original sentence is simpler:

• 这 = this
• not 这个原因 = this reason

That difference matters.

It is perfectly natural and can be used in both speech and writing.

It sounds slightly more careful or emphatic because of 真正, but not overly formal.

For example, in conversation someone might say:

• 这才是真正的原因。 = This is the real reason.
• 这就是真正的原因。 = This is exactly the real reason.

The basic sentence 这是真正的原因。 is clear, standard, and natural.