Learn Hardcore Chinese: tā bú fùxí de yuányīn shì tài lèi le, jiéguǒ kǎoshì méi kǎohǎo. - The reason he didn’t review was that he was too tired, and as a result he didn’t do well on the exam.

Before a 4th tone: 不 → bú
- e.g. 不对 → bú duì, 不好 → bú hǎo (here 好 is 3rd tone, so no change; but with 4th tone we see it clearly: 不贵 → bú guì)
In other cases, it stays bù.

Why is it 不复习 in the first part, but 没考好 in the second part?

不 and 没 both mean “not,” but they are used differently:

不: usually for habitual, general, or future actions, or for refusal / intention.
- 他不复习 = He doesn’t review / he chooses not to review (describing his behavior or decision, not a single completed event).
没: usually for past / completed actions that didn’t happen or didn’t succeed.
- 没考好 = did not do well (the result did not occur in that exam).

So:

不复习 = his (ongoing / typical or intentional) not-reviewing.
没考好 = in that exam, the result (“doing well”) did not happen.

Using 不考好 would sound wrong here, because the exam result is a specific event that failed to be achieved → use 没.

What does 的 do in 不复习的原因?

Here 的 links a describing phrase to a noun, turning the phrase into a kind of “adjective clause” or relative clause:

不复习的原因
- Literally: “the reason (that he) doesn’t review”

Structure:

[Verb phrase] + 的 + Noun
- 不复习的原因 = the reason for not reviewing
- 他来的时间 = the time when he comes
- 我买的书 = the book that I bought

So 不复习的 is describing 原因 (“reason”).

What is the function of 是 in 不复习的原因是太累了? Can it be left out?

Here 是 is the copula “to be”, linking 原因 (reason) with 太累了 (too tired):

不复习的原因是太累了
- The reason (why he didn’t review) is (that he was) too tired.

This is a common pattern:

A 的原因是 B = The reason for A is B

You could sometimes say more loosely:

他不复习，因为太累了。
“He didn’t review because he was too tired.”

But inside the sentence you gave, if you keep the pattern A 的原因是 B, you normally keep 是.
Dropping 是 here would sound incomplete or ungrammatical.

Why is it 太累了 and not just 太累? What does 了 mean here?

In 太累了, 了 is a sentence-final 了, often showing:

a new situation, or
a stronger, more immediate feeling (“now it’s too much / I really am too tired”).

Compare:

太累。
- Grammatically possible, but feels like a bare description: “(It’s) too tiring / I’m too tired.” (more neutral, less natural as a standalone statement)
太累了。
- Much more natural in speech: “(I’m) so tired now / I’m really too tired.”

In your sentence:

原因是太累了
implies “the reason is (that he was) really too tired (at that time / in that situation).”

Without 了, it would still be understandable, but 了 makes it sound more natural and expressive.

What does 结果 mean here, and where can it appear in the sentence?

结果 (jiéguǒ) here means “as a result / as a consequence” and works like a conjunctive adverb linking the two clauses:

……太累了，结果考试没考好。
- “…was too tired, as a result he didn’t do well in the exam.”

Common positions:

Usually at the beginning of the second clause:
- 他没复习，结果考试没考好。
It can also sometimes move slightly, but the natural and most common spot is where you see it: after a pause/comma, starting the result clause.

It emphasizes the cause → effect relationship.

In 考试没考好, why do we have 考试 and 考好 together? Isn’t that repeating 考?

Yes, 考 appears twice conceptually, but they play different roles:

考试 (kǎoshì) is a noun: “exam / test”.
考好 (kǎo hǎo) is a verb + result complement:
- 考 = to take an exam / to test
- 好 = well / successfully (as a result)

Structure here:

考试没考好
Literally: “As for the exam, (he) didn’t test-well.”

Chinese often puts the object before a verb + result complement:

饭没吃完。 = The meal wasn’t finished (lit. “meal not eat-finish”).
作业没写好。 = The homework wasn’t written well.

Similarly:

考试没考好。 = The exam (he) didn’t take well → He didn’t do well in the exam.

What exactly does 好 add in 考好?

好 here is a result complement, not just the adjective “good”. It shows the success / satisfactory completion of the action:

考好 = “to succeed in taking the exam / to do well in the exam.”

Compare result complements:

做好 = to do something well / finish doing it properly
写好 = to write something well / finish writing it properly
记住 = to memorize firmly (“remember-complete”)

So:

考好 = to achieve a good result in the exam
没考好 = failed to achieve a good result → did not do well.

Could we say 他没复习 instead of 他不复习? What’s the difference in meaning?

Yes, both are possible, but they focus on different things:

他不复习
- Often describes a habit / attitude / general behavior:
  - “He doesn’t review (as a rule / by choice).”
- In your sentence, 不复习的原因 feels like “the reason why he (chose to) not review / doesn’t review.”
他没复习
- Refers to a specific instance in the past:
  - “He didn’t review (this time / on that occasion).”

If you change the sentence to:

他没复习，是因为太累了，结果考试没考好。
It would mean: “He didn’t review (that time) because he was too tired; as a result he didn’t do well on the exam.”

Your original sentence with 不复习的原因 sounds slightly more general or explanatory: “the reason for his not reviewing (in that context).”

Is 不复习的原因是太累了 the only way to say “The reason he didn’t review is that he was too tired”?

No, there are several natural alternatives with slightly different structures and tone. For example:

他因为太累了，所以没复习。
- “Because he was too tired, he didn’t review.”
- Uses the common 因为…所以… pattern.
他没复习，是因为太累了。
- “He didn’t review; (the reason) is that he was too tired.”
- Puts the result first, then explains the reason.
他不复习，是因为太累了。
- Similar to your sentence but without turning it into a “X 的原因是 Y” pattern.

Your version:

他不复习的原因是太累了
sounds very explicit and slightly more formal / written because of the X 的原因是 Y structure.

What overall structure does this sentence have? How are the two parts connected?

The sentence is made of two clauses with a clear cause → result relationship:

Cause / explanation clause
- 他不复习的原因是太累了，
- “The reason he doesn’t/didn’t review is that he was too tired,”
Result clause, introduced by 结果
- 结果考试没考好。
- “As a result, he didn’t do well on the exam.”

Pattern:

[Reason clause]，结果 [Result clause]。

This is a very common way to narrate events in Chinese:

他没带伞，结果被雨淋了。
- He didn’t bring an umbrella; as a result, he got rained on.
他们出发晚了，结果没赶上火车。
- They left late; as a result, they missed the train.