Learn Hardcore Chinese: Jìrán nǐ zhème lèi, jiù jīntiān zài sùshè hǎohāo xiūxi ba. - Since you’re so tired, then you should rest well in the dorm today.

It assumes A is already a known or accepted fact, and B is the natural consequence, decision, or suggestion.
In this sentence: 既然你这么累，就今天在宿舍好好休息吧。 → “Since you’re so tired, then (you should) have a good rest in the dorm today.”

Compared with 因为 … 所以 …:

因为 A，所以 B is more neutral and factual: “because A, therefore B”.
既然 … 就 … carries more of a “given that that’s the case, let’s do this” feeling, often used when giving advice, suggestions, or decisions, not just explaining a reason.

Is 就 necessary in this sentence? What does it add to the meaning?

You can say the sentence with or without 就:

With 就: 既然你这么累，就今天在宿舍好好休息吧。
Without 就: 既然你这么累，今天在宿舍好好休息吧。

就 here adds the nuance of “then / in that case / just”, highlighting that the action in the second part is the natural and immediate reaction to the first part.

Without 就, the meaning is still clear, but it sounds a bit more plain; with 就, it feels more connected and decisive, like: “Well then, in that case, rest well in the dorm today.”

Why is 今天 placed between 就 and 在宿舍? Can we move 今天 to other positions?

Chinese generally prefers the order: time → place → action.
In the sentence’s second part we have:

就 + 今天 + 在宿舍 + 好好休息

So 今天 (time) before 在宿舍 (place) follows a common pattern. You can move 今天 a bit, and several versions are natural:

既然你这么累，今天就在宿舍好好休息吧。
既然你这么累，就在宿舍今天好好休息吧。 (OK, but the first one is more common.)

The most natural/common for everyday speech would probably be:

既然你这么累，就在宿舍好好休息吧。 (with 今天 understood from context)
or
既然你这么累，今天就在宿舍好好休息吧。

Why isn’t there a second 你 in the second clause (like “你今天就…”)? Is it wrong to add it?

Chinese often omits repeated subjects when they are obvious from context.

Full version (possible, but heavier):
- 既然你这么累，你今天就在宿舍好好休息吧。
Natural version with omission:
- 既然你这么累，就今天在宿舍好好休息吧。

Both are grammatically correct.
Omitting the second 你 sounds more smooth and conversational, because it avoids unnecessary repetition. Adding it makes the sentence a bit more emphatic or formal-sounding, but not wrong.

What does 这么 mean in 你这么累? How is it different from 很 or 那么?

这么 means “so / this (much)”, expressing a high degree and often some emotion or surprise:

你这么累 → “You’re so tired / this tired.”

Comparison:

很累 – “very tired”, more neutral; just stating degree.
这么累 – “so tired”, often used when the speaker feels something about it (concern, surprise, etc.), and usually refers to something close to the speaker (right now, in front of me).
那么累 – “that tired / so tired (like that)”, often when referring to something a bit further away in time, space, or perspective, e.g. 他那么累.

In this sentence, 这么累 fits the emotional, caring tone: “Since you’re so tired…”

Why is it 好好休息 and not 好好地休息? Is 地 missing?

In modern spoken Chinese, many common adverb-like words are used without 地, especially when they are:

Reduplicated adjectives: 好好, 慢慢, 认真, etc.
Very frequent, fixed expressions.

So:

好好休息 and 好好地休息 are both grammatically correct.
好好休息 is more natural in everyday speech.
好好地休息 can sound a bit more formal or emphasized, like “rest really properly/well,” but the difference is small.

In this sentence, 好好休息 is the standard colloquial form.

What exactly does the reduplication in 好好 mean? Why not just 好休息?

Reduplication (好 → 好好) often adds a sense of:

Doing something properly / nicely / thoroughly,
While keeping the tone friendly or gentle, not harsh.

好休息 is not idiomatic; 好好 here acts like an adverbial expression:

好好休息 ≈ “rest properly / have a good rest / rest well.”

Other similar patterns:

慢慢走 – walk slowly, take your time.
好好想想 – think it over carefully.

So 好好 is not just “good”, it’s more like “properly, nicely, thoroughly” in a warm, encouraging way.

What is the function of the particle 吧 at the end of the sentence?

Sentence-final 吧 softens the tone and turns the sentence into a suggestion, advice, or mild request instead of a direct command.

好好休息。 – “Rest well.” (can sound like a straightforward command or instruction)
好好休息吧。 – “Why don’t you rest well.” / “Go rest well.” (gentler, more caring)

In this sentence, 吧 matches the context of concern and suggestion: the speaker is not ordering, but encouraging the listener to rest.

Why is it 在宿舍 and not 到宿舍 or 回宿舍? And can we say 在宿舍里 instead?

在宿舍 means “at/in the dorm”, describing location, not movement.
到宿舍 / 回宿舍 emphasize movement (“go to the dorm” / “go back to the dorm”), which is not the main point here. The focus is on resting where you are (or will be), not the act of going.

You can say 在宿舍里好好休息 as well:

在宿舍里 → “inside the dorm”, slightly more explicit about “inside”.
In many everyday contexts, Chinese speakers just say 在宿舍, and the idea of “inside” is understood, so 里 is often omitted.

So: 在宿舍好好休息 is perfectly natural and common.

Could we replace 既然 with 如果 here? For example, 如果你这么累，就今天在宿舍好好休息吧. What’s the difference?

You can say 如果你这么累，就今天在宿舍好好休息吧, but the nuance changes:

如果 = “if”, expressing a condition or hypothesis. It doesn’t assume the condition is already true.
既然 = “since / now that”, expressing something the speaker treats as an established fact.

In the original context, the speaker clearly believes “you are so tired” is already true, so 既然 is more natural.

Using 如果 can sound more like:

“If you’re (indeed) so tired, then rest in the dorm today,”
which is more tentative or conditional.

Why isn’t there any tense marker like 会, or aspect marker like 了, with 休息 here?

Chinese usually relies on context and time words (like 今天) for tense, rather than verb changes.

In this sentence:

今天 and the suggestion form with 吧 already make it clear this is about what you should do today (in the near future).
会休息 would sound like “will rest / be able to rest”, which is unnecessary here.
休息了 would describe a completed rest (“have rested / rested already”), which doesn’t fit a suggestion for a future action.

So the simple 休息 is the natural choice: it fits a general instruction/suggestion about what to do today.

Can we drop 今天 and just say 既然你这么累，就在宿舍好好休息吧? How does that change the meaning?

Yes, and that version is very natural:

既然你这么累，就在宿舍好好休息吧。

Without 今天, the sentence means “Since you’re so tired, then (just) rest well in the dorm”.

In many real-life situations, listeners will automatically understand “today / now” from context.
Adding 今天 makes the time frame explicit, emphasizing “today, specifically”:
- 既然你这么累，就今天在宿舍好好休息吧。 → “Since you’re so tired, then (at least) today, rest well in the dorm.”

So both are correct; the version without 今天 is slightly more general, while the original one puts a bit more focus on today.