Learn Hardcore Chinese: xiàyǔ de shíhou, tā chángcháng zài jiā kàn shū. - When it rains, she often stays home and reads.

的: links a modifier to a noun (下雨的时候).
地: makes an adverb before a verb (高兴地看书).
得: follows a verb to introduce a complement (看得很认真). Don’t drop 的 here; 下雨时候 sounds unnatural.

Can I say 下雨时 instead of 下雨的时候?

Yes. 下雨时 is a shorter, slightly more formal/literary version. In everyday speech, 下雨的时候 is more common. Both are correct.

Do I need 在 before 下雨的时候? Is 在下雨的时候 okay?

Both 下雨的时候 and 在下雨的时候 are acceptable. 在 adds a “during/at the time of” feel but is usually unnecessary; the shorter form is more natural in speech.

Can I move 下雨的时候 after the subject? (e.g., 她下雨的时候常常在家看书)

Avoid that. 她下雨的时候… literally reads as “when she rains,” which is semantically wrong. Keep the time clause first: 下雨的时候，她常常在家看书。 If you really want it after the subject, use a preposition: 她在下雨的时候常常在家看书 (grammatical but wordy).

Where does 常常 go? Could I say 她在家常常看书?

Both 她常常在家看书 and 她在家常常看书 are fine.

Default placement: subject + frequency adverb + location + verb-object (她常常在家看书).
Putting 常常 after the location (她在家常常看书) slightly emphasizes the location (“at home, she often reads”). Don’t put the location at the very end: 她常常看书在家 is unnatural.

Is 常常 the same as 经常 or 常?

All can mean “often.”

常常: neutral, common in speech.
经常: slightly more formal; “regularly.”
常: concise; can sound a bit literary depending on context. They’re interchangeable here: 她经常/常常/常在家看书.

How do we know this is habitual without tense markers?

Chinese doesn’t mark tense like English. 常常 and context indicate a repeated habit. For past habits, add time words: 过去/以前. For a typical tendency, add 会: 下雨的时候，她会在家看书.

Why is 在 before 家? Could 在 also be the progressive “be doing” here?

Here 在 marks location: 在家 = “at home.” The progressive 在 comes directly before a verb (e.g., 在看书). Don’t use both: 她常常在家在看书 sounds awkward. To show an ongoing action now: 她在家看书呢 or 她正在看书.

Does 看书 mean “read a book” or just “read”?

Both. 看书 means “to read (books)” as an activity, not necessarily one specific book. To specify a particular book or quantity, add a measure word: 看一本书. Note: 读书 can also mean “to read,” and additionally “to study; be in school.”

Do I need a measure word before 书 here?

No. It’s a generic activity. Use a measure when specifying number or a specific book: 一本书, 那本书. Generic activities like 看书, 看电视, 上网 don’t take a measure word.

What’s the difference between 在家 and 在家里?

Both mean “at home.” 在家 is the default. 在家里 can feel a bit more explicit (“inside the home”) or emphatic. They’re interchangeable here.

Can I omit 时候 and just say 下雨，她常常在家看书?

In careful writing, keep 的时候. In casual speech, people may topicalize with a pause or particle: 下雨啊，她常常在家看书. Without that, 下雨，她… feels abrupt.

Is 下雨了的时候 acceptable?

No. Use 下雨的时候 for “when it’s raining,” or 下雨了以后 for “after it starts raining.” To emphasize ongoing rain during that time, you can say 下着雨的时候.

Can I use 一…就… to mean “whenever it rains, she (then) reads”?

Yes:

一下雨，她就在家看书。
For a tendency: 一下雨，她就会在家看书。 Don’t combine with 常常 (e.g., “就常常”)—一…就… already implies regularity.

What’s the difference among 下雨, 下着雨, and 下雨天?

下雨: “to rain” (neutral).
下着雨: emphasizes the action is ongoing (“it’s raining”).
下雨天: “a rainy day” (a time noun). Example: 下雨天她常常在家看书。

Why not say 雨下的时候?

Although grammatical, it’s not idiomatic. Weather expressions typically use verb–object order with the verb first: 下雨 (“fall rain”). The natural phrase is 下雨的时候, not 雨下的时候.