nánguài nǐ jīntiān zhème lèi, yuánlái zuótiān wǎnshang méi shuìhǎo.

难怪 means no wonder or it’s no surprise that.

In this sentence, 难怪你今天这么累 means No wonder you’re so tired today.

A few useful points:

• 难怪 is often used when you learn the reason for something and suddenly it makes sense.
• It can introduce a whole clause:
  • 难怪你今天这么累。
• It can also stand alone in conversation:
  • A: 我昨天只睡了三个小时。
  • B: 难怪！
  • Meaning: No wonder!

So here, the speaker first notices the result, then realizes the cause.

Here, 原来 means so that’s why, it turns out, or as it turns out.

In this sentence:

• 原来昨天晚上没睡好
• literally: It turns out you didn’t sleep well last night

This use of 原来 introduces newly discovered information that explains something.

Be careful: 原来 can also mean originally or former in other contexts. For example:

• 原来我以为他不来。
  • Originally / at first, I thought he wasn’t coming.

But in your sentence, it is the it turns out meaning.

这么 means so, this, or to this degree.

So:

• 这么累 = so tired

It is a degree word placed before an adjective.

Common patterns:

• 这么 + adjective
  • 这么累 = so tired
  • 这么忙 = so busy
• 那么 + adjective
  • also means so or that, often a little more detached
• 很 + adjective
  • often just marks the adjective naturally, sometimes not as strong as 这么

Compare:

• 你今天很累。 = You’re tired today.
• 你今天这么累。 = You’re so tired today.

Here, 这么 helps show that the speaker has noticed an unusual degree of tiredness.

Because in Mandarin, adjectives can act as predicates by themselves.

So:

• 你今天累 = You’re tired today.
• 你今天这么累 = You’re so tired today.

You do not say:

• 你今天是累 ❌

English usually needs to be before adjectives, but Mandarin usually does not.

This is a very common difference between English and Chinese. Adjectives like 累, 忙, 高兴, 漂亮 can directly describe the subject without 是.

Yes, 原来你昨天晚上没睡好 is completely natural.

In the sentence you were given, the subject 你 is omitted after 原来 because it is already obvious from context.

So the full idea is:

• 难怪你今天这么累，原来你昨天晚上没睡好。

Chinese often leaves out subjects when they are clear and repeating them would sound unnecessary.

This kind of omission is very common in everyday Mandarin.

没 is usually used to negate past actions or situations.

Since 昨天晚上 refers to the past, 没 is the natural choice:

• 昨天晚上没睡好 = did not sleep well last night

Why not 不?

• 不 is more often used for:
  • habits
  • general statements
  • future actions
  • intentions

Compare:

• 我昨天没睡好。 = I didn’t sleep well yesterday.
• 我晚上不喝咖啡。 = I don’t drink coffee at night.

Also, with many verb-result forms like 睡好, 做好, 看懂, 吃完, 没 is especially common in negative past statements.

They are related, but not exactly the same.

睡好

This is verb + result complement:

• 睡 = sleep
• 好 = well / satisfactorily / successfully

So 睡好 means something like:

• to get good sleep
• to sleep well enough
• to have slept properly

In negative form, 没睡好 is extremely common and natural.

睡得好

This uses the 得 complement pattern and focuses more directly on the quality of the action:

• 睡得好 = sleep well

Compare:

• 我昨天没睡好。 = I didn’t get good sleep yesterday.
• 我最近睡得不好。 = I haven’t been sleeping well recently.

In everyday speech, 没睡好 is one of the most common ways to say didn’t sleep well.

Because time expressions in Mandarin usually come before the verb.

A very common word order is:

• subject + time + verb phrase

So:

• 你昨天晚上没睡好
• you + last night + didn’t sleep well

This is normal Mandarin word order.

More examples:

• 我今天很忙。
• 他明天去北京。
• 我们上个月见过。

Time words often come early in the sentence, usually before the main verb.

This is a very useful pattern for expressing result first, explanation second.

Structure:

• 难怪 + result
• 原来 + explanation/cause

In your sentence:

• 难怪你今天这么累
  • No wonder you’re so tired today
• 原来昨天晚上没睡好
  • It turns out you didn’t sleep well last night

So the speaker first notices something, then gives the reason after realizing it.

This pattern is common in conversation because it sounds very natural and expressive.

Yes, some word-order variation is possible.

Both of these sound natural:

• 难怪你今天这么累。
• 难怪今天你这么累。

The difference is small:

• 难怪你今天这么累 slightly highlights you
• 难怪今天你这么累 slightly brings today forward

Both are understandable and natural.

Also, after 原来, you can say either:

• 原来昨天晚上没睡好
• 原来你昨天晚上没睡好

The second one is a little more explicit.

累 can describe both physical and general tiredness.

It can mean:

• physically tired
• worn out
• exhausted
• mentally drained, depending on context

Examples:

• 今天走太多路了，好累。
  • I walked too much today. I’m so tired.
• 工作了一整天，真累。
  • I worked all day. I’m really tired.

So in your sentence, 累 can naturally mean the person looks tired overall, not just physically.

晚上 covers the general evening-night period, and in many sentences it is best translated naturally rather than word-for-word.

So:

• 昨天晚上没睡好

is most naturally:

• didn’t sleep well last night

Even though 晚上 can also refer to evening, in this sentence the natural English translation is last night, because sleeping is associated with nighttime.

So this is a good example where natural translation matters more than matching each word exactly.