dōngtiān yàoshi xiàxuě le, tā bú shì zuò dìtiě, jiù shì zǎodiǎnr huíjiā.

Yes. 冬天 here functions as a time word, and in Chinese, time words often go near the beginning of the sentence without needing a preposition like 在.

So:

• 冬天 = winter
• at the start of a sentence, it often means in winter / during winter

This is very common in Chinese:

• 明天我去。 = I’m going tomorrow.
• 晚上他不出门。 = He doesn’t go out at night.

So 冬天要是下雪了... naturally means If it snows in winter...

Yes. 要是 means if, and it introduces a condition.

In this sentence:

• 要是下雪了 = if it snows / if it ends up snowing

Common Chinese words for if include:

• 如果 — common, slightly more formal
• 要是 — common in speech, very natural
• 假如 — more formal/literary

So 要是 is a very normal conversational way to say if.

Here, 了 does not simply mean past tense. It often marks a new situation or change of state.

So 下雪了 suggests:

• it has started snowing
• it turns out to snow
• snow comes / appears as a situation

In a conditional sentence like this, 要是下雪了 means something like:

• if it snows
• if it starts snowing
• if there is snow

This is a very common use of 了 in spoken Chinese.

Not necessarily. That is a very common misunderstanding for English speakers.

Chinese 了 does not work exactly like English past tense. In 要是下雪了, the idea is conditional, not necessarily past.

It can mean:

• if it snows
• if it starts snowing
• if snow comes

So even though 了 is present, the whole clause can still refer to a future or hypothetical situation because of 要是.

不是……就是…… is a fixed pattern meaning either...or...

So:

• 她不是坐地铁，就是早点儿回家。 = She either takes the subway, or goes home a bit early.

It does not literally mean:

• she is not X, then is Y

Instead, treat 不是……就是…… as one chunk with a special meaning:

• either A or B

Other examples:

• 他不是喝茶，就是喝咖啡。 = He either drinks tea or coffee.
• 周末我不是在家，就是出去。 = On weekends I’m either at home or out.

Because of tone sandhi.

Normally:

• 不 is pronounced bù (4th tone)

But when 不 comes before another 4th-tone syllable, it changes to bú (2nd tone).

Since:

• 是 shì is 4th tone

we say:

• 不是 = bú shì

This is a pronunciation change, not a change in meaning.

Yes, literally 坐 often means sit, but with transportation it commonly means ride / take.

So:

• 坐地铁 = take the subway
• 坐飞机 = take a plane
• 坐火车 = take a train
• 坐公交车 = take the bus

This is a very common usage in Chinese. English learners often find it strange at first, but it is completely normal.

Because the subject is shared by both parts of the either...or... structure.

So:

• 她不是坐地铁，就是早点儿回家。

means:

• She either takes the subway or goes home early.

Chinese often avoids repeating the subject when it is already clear.

You could say:

• 她不是坐地铁，就是她早点儿回家。

but that sounds unnecessarily repetitive in normal speech.

早点儿 means a bit earlier or somewhat early.

Breakdown:

• 早 = early
• 点 = a little / a bit
• 儿 = an -r ending, common in northern Mandarin

So 早点儿回家 means:

• go home a bit earlier
• go home early

This 点(儿) pattern is common:

• 慢点儿 = a little slower
• 快点儿 = a little faster / hurry up
• 晚点儿 = a little later

The 儿 is an erhua ending, a rhotic -r sound often heard in northern Mandarin, especially Beijing-style speech.

So 早点儿 is often pronounced roughly like:

• zǎodiǎnr

In many regions, people may say something closer to:

• 早点
• 早一点

So yes, you should recognize it, but pronunciation varies by accent. If your learning materials use 儿, it is good to learn it, but don’t worry if you also hear forms without a strong -r sound.

Because Chinese often leaves future meaning to context.

Here, the future or habitual meaning is already clear from:

• 冬天 = in winter
• 要是 = if
• the overall situation

So Chinese does not need a separate will in many cases.

English often requires a future idea more explicitly, but Chinese frequently does not.

If you added 会, it might sound like:

• 冬天要是下雪了，她不是会坐地铁，就是会早点儿回家。

But this is less natural here. The original sentence is smoother.

It most naturally suggests a habitual or typical reaction:

• In winter, if it snows, she either takes the subway or goes home early.

That sounds like a pattern of behavior, not just one single event.

Chinese often allows this kind of general statement without adding special markers like usually. The context gives that meaning.

The sentence follows a very common Chinese order:

time + condition + subject + main statement

So here:

• 冬天 = time
• 要是下雪了 = condition
• 她 = subject
• 不是坐地铁，就是早点儿回家 = main statement

That makes the structure:

• In winter, if it snows, she either takes the subway or goes home a bit early.

This kind of ordering is extremely common in Chinese.

Yes. That would be completely grammatical.

For example:

• 冬天如果下雪了，她不是坐地铁，就是早点儿回家。

This means the same thing.

The difference is mostly tone and style:

• 要是 = very natural, conversational
• 如果 = also common, slightly more neutral or formal

Yes. 早一点回家 is also correct.

Compare:

• 早点儿回家
• 早一点回家

Both mean go home a bit earlier.

The first is often more colloquial and compact in speech. The second may feel slightly more transparent to learners because it clearly shows 一点.

So both are good, and native speakers use both.