tā zuótiān méi shuì hǎo, jīntiān shàngkè de shíhou chàdiǎnr shuìzháo.

Because 没 is the normal negation for a past event or for something that did not happen.

• 没睡好 = didn’t sleep well
• 不睡好 would not fit here. 不 is usually used for habits, general truths, intentions, or future actions.

So in this sentence, 昨天没睡好 is correct because it talks about what happened yesterday.

Here 睡好 means to sleep well or to get good sleep.

The 好 after the verb is not just the adjective good in the usual English sense. It works like a result complement, showing that the action reached a satisfactory result.

So:

• 睡好 = sleep well / get enough good sleep
• 没睡好 = didn’t sleep well

Both can mean didn’t sleep well, but the feeling is slightly different.

• 没睡好 often suggests the speaker did not achieve the result of good sleep.
• 睡得不好 describes the quality of the sleep more directly: slept poorly.

So:

• 她昨天没睡好 = she didn’t get a good night’s sleep
• 她昨天睡得不好 = she slept badly yesterday

In many situations, both are natural.

Because in Mandarin, words like 好 can come after a verb as a complement.

Compare:

• 好睡 would usually mean easy to sleep in/on or comfortable for sleeping, depending on context.
• 睡好 means to sleep well or to sleep and get a good result.

So 睡好 is a verb + complement structure, not an adjective modifying the verb the way English does with well.

Mandarin does not need a past-tense ending like English does.

Past time is often shown by:

• a time word, such as 昨天
• context
• or sometimes 了

Here, 昨天 already makes the time clear, and 没 naturally negates a past/completed action. So 她昨天没睡好 is complete and natural without 了.

Literally, it means at the time of attending class.

In natural English, it means:

• when she was in class
• during class
• while class was going on

Breakdown:

• 上课 = to attend class / to be in class / to have class
• 的时候 = when / at the time when

So 今天上课的时候 means when she was in class today.

After a verb phrase, 的 is usually used before 时候.

So these are natural:

• 吃饭的时候 = when eating
• 下雨的时候 = when it rains
• 上课的时候 = when in class

The 的 links the action phrase to 时候.

You will sometimes see fixed expressions without 的, but for learners, verb phrase + 的时候 is the safest pattern.

Because 的时候 already expresses that meaning.

• 上课的时候 already means when/during class
• You do not need a separate word for during

Sometimes Mandarin can use 在……的时候, but here that would usually sound unnecessary.
So 今天上课的时候 is already natural and complete.

差点儿 means almost or nearly.

So:

• 差点儿睡着 = almost fell asleep

It shows that something came very close to happening.

Normally, 差点儿睡着 means she almost fell asleep, but didn’t quite.

So the usual understanding is:

• she was very sleepy in class
• she came close to falling asleep
• but she did not fully fall asleep

In everyday speech, 差点儿 can sometimes be discussed as a tricky word, especially with negative events, but here the normal interpretation is simply she almost fell asleep.

These are different ideas:

• 睡觉 = to sleep / go to sleep in a general sense
• 睡好 = to sleep well
• 睡着 = to fall asleep, or to be asleep

In this sentence, 睡着 means fall asleep.

So:

• 她差点儿睡着 = she almost fell asleep
• 她没睡好 = she didn’t sleep well

One talks about the quality of sleep, and the other talks about entering the asleep state.

Because 着 has multiple pronunciations in Mandarin, and this is one of them.

Common readings include:

• zhe: aspect marker, as in 看着
• zhuó: less common in everyday beginner material
• zháo: often used in result meanings, such as 睡着

In 睡着, 着 is pronounced zháo and indicates that the result was reached: the person actually fell asleep.

Because Mandarin often omits repeated subjects when they are already clear from context.

So this sentence is understood as:

• 她昨天没睡好，今天上课的时候差点儿睡着。

The second clause still refers to 她, even though the subject is not repeated.

You could say 她今天上课的时候差点儿睡着, but leaving out the second 她 is very natural.

Time words in Mandarin usually come before the verb phrase.

So:

• 她 昨天 没睡好
• 今天 上课的时候 差点儿睡着

That is very typical Mandarin order:

• subject
• time
• verb phrase

You can sometimes move time expressions for emphasis, such as:

• 昨天她没睡好

But the sentence as given is completely natural.

In some contexts, 上课 can refer to having class, and depending on who the subject is, it may mean teaching or attending.

Here, because the sentence says she almost fell asleep 上课的时候, the natural reading is while she was in class or during class. In other words, she was a student in class, not the teacher giving the lesson.

Context decides the exact English translation.