Learn Hardcore Chinese: yīshēng shuō, rúguǒ jīntiān wǎnshang yòu fāshāo, jiù gǎnjǐn lái yīyuàn. - The doctor said that if she gets a fever again tonight, she should come to the hospital right away.

Breakdown of yīshēng shuō, rúguǒ jīntiān wǎnshang yòu fāshāo, jiù gǎnjǐn lái yīyuàn.

Why is there no subject like 你 in 如果今天晚上又发烧，就赶紧来医院?

Chinese often omits subjects when they are obvious from context.

In this sentence, the doctor is clearly speaking to a specific person, so 你 is understood:

But in natural speech, repeating 你 is often unnecessary, so it gets dropped.

What does 又 mean here?

又 means again.

So 又发烧 means get a fever again or have a fever again.

It suggests the person has already had a fever before, and the doctor is talking about it happening one more time tonight.

Why is 又 used instead of 再?

This is a very common question.

Both 又 and 再 can relate to repetition, but they are used differently:

又 often refers to something happening again, especially when it has already happened before and the speaker sees it as a repeated event.
再 is more commonly used for doing something again in the future.

Here, 又发烧 sounds natural because the doctor is talking about the return of a symptom the patient has already had.

A learner-friendly way to think about it:

In real life, this distinction is not always perfectly simple, but in this sentence 又发烧 is very natural.