Learn Hardcore Chinese: zhīdào nǐ zài jiā, wǒ jiù fàngxīn le. - Knowing that you are at home, I feel relieved.

This is also correct, just a bit more explicit. Dropping 我 makes the sentence slightly more casual and natural in conversation, especially if it’s already clear that we’re talking about what I know or feel.

Grammatically, what is the structure of 知道你在家? Is 你在家 a separate clause?

Yes. The structure is:

知道 – verb: to know
你在家 – a clause acting as the object of 知道

Inside 你在家:

你 – subject: you
在 – verb: to be (located) at
家 – location: home

So 知道你在家 means “to know (that) you are at home.”
Chinese allows a full clause (你在家) to function directly as the object of verbs like 知道, 觉得, 以为, etc., without a word like “that” in English.

Why is it 你在家 and not 你是在家? Don’t we usually need 是 for “to be”?

In Chinese:

是 is used when the predicate is a noun or noun phrase:
- 他是老师。 – He is a teacher.
- 这是我的书。 – This is my book.
在 is used when the predicate is a location:
- 他在学校。 – He is at school.
- 你在家。 – You are at home.

You normally don’t use 是 + 在 together in this kind of simple location sentence. So:

你在家 – correct
你是在家 – possible only in special emphatic or contrastive contexts, like:
- 你是在家，可是心不在家。
  You are at home, but your heart isn’t.
  Here 是 adds contrast/emphasis.

In the given sentence, simple 你在家 is the normal, neutral form.

What exactly does 就 (jiù) do in 我就放心了? Can I leave it out?

Here 就 marks a result that follows directly and naturally from the first part of the sentence. Rough meaning:

知道你在家，我就放心了。
Once / as soon as / since I know you’re at home, I (then) feel relieved.

Functions of 就 here:

Shows cause → effect:
- Cause: 知道你在家
- Effect: 我放心了
Implies immediacy or readiness of the result:
- The moment I know, I’m relieved.

If you leave it out:

知道你在家，我放心了。

This is still grammatical and understandable, but:

With 就: feels more like “then / immediately / naturally, I’m relieved.”
Without 就: just states two facts in sequence, with weaker cause-effect emphasis.

So 就 makes the connection between the two parts clearer and more natural.

What does 放心 (fàngxīn) literally mean, and is it a verb or an adjective?

放心 is a verb (or verbal expression), literally:

放 – to put, to let go
心 – heart, mind

So 放心 is “to let go of (one’s) heart/mind”, which means:

to stop worrying / to feel at ease / to be relieved / to rest assured

Usage:

你放心吧。 – Don’t worry / Just relax.
我现在可以放心了。 – Now I can be at ease.
这件事交给他，我很放心。 – I feel very reassured leaving this to him.

It’s not an adjective like English “relaxed”; it behaves as a verb meaning “be/feel relieved; stop worrying.” In English we often translate it as “I’m relieved,” but in Chinese it’s more like “I relax/let go mentally.”

What does the sentence‑final 了 in 我就放心了 do? Is it past tense?

The sentence‑final 了 (at the end of 放心了) usually marks a change of state or a new situation, not simple past tense.

Here it suggests:

Previously, I was not at ease / was worried.
Now (after knowing you are at home), my state has changed to being relieved.

So:

我就放心了。 ≈ “Then I become relieved / I can be at ease now.”

It does not strictly mean past tense; Chinese doesn’t mark tense the same way English does. Time is inferred from context; 了 focuses on the arrival of a new situation.

Does this sentence talk about the past, the present, or a general situation?

It can express different time frames depending on context:

Past event (very common):
- When I heard you were home, I felt relieved.
  Context: someone describing what happened earlier.
Present/situational:
- Now that I know you’re at home, I feel relieved (right now).
General rule / repeated situation (less likely but possible with added time words):
- 每次知道你在家，我就放心了。
  Every time I know you are at home, I feel relieved.

The Chinese sentence itself is neutral. Listeners decide the time frame from context or added time expressions like 刚才, 以后, 每次, etc.

Why is there only a comma (，) between the two parts? Shouldn’t we need words like 因为 or 所以 to show cause and effect?

Chinese often uses just a comma between two clauses where English would use “because”, “so”, or “when”.

Pattern here:

[Reason / condition]， [Result].

In this sentence:

知道你在家， – When / Once / Since I know you are at home,
我就放心了。 – I’m then relieved.

You could make the cause–effect even more explicit:

因为知道你在家，我就放心了。 – Because I know you’re at home, I feel relieved.
知道你在家，所以我就放心了。 – I know you’re at home, so I feel relieved.

But the original, with just a comma and 就, is completely natural. The combination [clause]，就… already strongly implies “when/once/then.”

Could I say 一知道你在家，我就放心了? How is that different from the original?

Yes, that is natural and common:

一知道你在家，我就放心了。

Here 一…就… is a set pattern meaning “as soon as …, (then) …”

一 – “as soon as / the moment (that)”
就 – “then / immediately / naturally”

So:

知道你在家，我就放心了。 – When / Once I know you’re at home, I’m relieved.
一知道你在家，我就放心了。 – The very moment I know you’re at home, I’m relieved.

The second version emphasizes immediacy more strongly: the relief happens instantly upon knowing.

What’s the difference between 放心 and 安心 or 安全? They all seem related to “safe” or “at ease”.

They are related but not interchangeable:

放心 (fàngxīn) – verb: to stop worrying; to feel relieved / reassured
- Focus: your feelings (worry → ease)
- 你到了给我打个电话，我才放心。
  Call me when you arrive; only then will I be at ease.
安心 (ānxīn) – verb/adjective: to feel calm; to settle down with peace of mind
- More about being calm and settled, sometimes long-term.
- 你在这里安心学习吧。 – Just study here with peace of mind.
- 我安心了。 – I feel at peace now (worry has subsided).
安全 (ānquán) – noun/adjective: safety; safe
- Objective safety, not your subjective worry.
- 这里很安全。 – It’s very safe here.
- 注意安全。 – Be careful / Pay attention to safety.

In the given sentence, we care about the speaker’s feeling of relief, so 放心 is the correct choice.

Could I say 知道你在家了，我就放心了 with an extra 了 after 家? What would change?

Yes, that’s possible and slightly changes the nuance:

知道你在家，我就放心了。
Emphasis: Knowing that you are at home (a general state).
知道你在家了，我就放心了。
The 了 after 在家 suggests a newly reached state:
- You have arrived home now / have made it home.
- Once I know you have gotten home, I’m relieved.

So:

Without the inner 了: knowing you are at home (in general) is enough to make me feel at ease.
With 在家了: the relief is tied to the completion of you getting home (e.g. after traveling).

Can I drop the first 知道 and just say 你在家，我就放心了? Does it mean the same thing?

You can say:

你在家，我就放心了。

This is also natural, but the meaning shifts slightly:

知道你在家，我就放心了。
Focus: my knowing that you’re home causes the relief.
→ Once I know you’re at home, I’m relieved.
你在家，我就放心了。
Focus: the fact that you’re at home is the condition for my relief.
→ As long as you’re at home, I’m relieved.

In everyday conversation, the difference is subtle and context-dependent, but grammatically:

Original: emphasizes the moment/act of finding out.
Shortened: emphasizes the situation itself (you being at home).

Is there any difference in meaning between 我就放心了 and 我就放心 (without 了)?

Yes, there is a nuance difference:

我就放心了。
- Emphasizes a change of state: “I become relieved / I can now be at ease.”
- Very natural in this context, because it contrasts the previous worry with the new relief.
我就放心。
- Grammatically possible, but sounds a bit incomplete or less natural here.
- Without 了, it can feel more like a habitual/general statement or like you’re describing a characteristic:
  - e.g. (in a different context) 你在就行，我就放心。 – If you’re here, I’m (always) reassured.

In this specific sentence, 我就放心了 is the most natural and idiomatic choice.