Learn Hardcore Chinese: Tā yí xiàkè jiù gēn tóngxué yìqǐ shàngwǎng. - As soon as class is over, she goes online with her classmates.

Pattern in this sentence:
- 一 + 下课 + 就…
- 下课 is the whole verb “to finish class”
- 一…就… = “as soon as … then …”
The verb + 一下 pattern you already know:
- e.g. 看一下 = “have a quick look”
- 想一下 = “think about it briefly”

So:

她一下课就… = “As soon as she finishes class, she …”
她下课以后想一下。 = “After class she wants to think (about it) a bit.”

In short, in the original sentence, 一 belongs with 就 (the 一…就… pattern), not with 下课.

Why is 一 pronounced yí here instead of yī?

This is a case of tone sandhi (tone change rules).

The basic tone of 一 is first tone (yī).
But when 一 comes before a fourth-tone syllable, it usually changes to second tone (yí).

In 一下课:

下课: xià (4th tone) + kè (4th tone)
一 comes right before 下 (xià), which is 4th tone.
So 一 changes from yī → yí.

That’s why the pinyin is yí xiàkè for 一下课 in this pattern.

What exactly does 就 do in this sentence? Could we just say 她一下课跟同学一起上网?

就 here emphasizes that the second action happens immediately or right after the first one.

她一下课就跟同学一起上网。
“As soon as she finishes class, she goes online with her classmates (right away).”

If you remove 就:

她一下课跟同学一起上网。

This is grammatically possible, but it sounds a bit off or incomplete to native speakers, because the 一…就… pattern is usually a pair. You normally want both 一 and 就.

More natural options:

她下课就跟同学一起上网。 (keep 就, drop 一)
她一下课就跟同学一起上网。 (keep the full 一…就… pattern)

So in this pattern, 就 is strongly expected; it makes the sentence sound natural and highlights the immediacy of the second action.

Why do we need both 跟 and 一起? Don’t they both mean “with / together”?

They are related, but they do different jobs:

跟 introduces the person you are with.
Think: “with (someone)”.
一起 describes how you do the action: together.

In the sentence:

跟同学 – with (her) classmates
一起 – together
上网 – go online

So 跟同学一起上网 = “go online together with her classmates”.

You could say:

她跟同学上网。 – “She goes online with her classmates.” (no 一起)
This is OK, but feels a bit less explicit about “together”.
她和同学一起上网。 – also fine (和 instead of 跟)

The combination 跟/和 + someone + 一起 is very common and very natural when talking about doing things with other people.

What’s the difference between 跟 and 和 here? Can I say 她一下课就和同学一起上网?

Yes, you can say:

她一下课就和同学一起上网。

Both are correct and natural. The differences are small:

跟 is slightly more colloquial / spoken in many regions.
和 is slightly more neutral and is common in both speech and writing.

In everyday conversation, 跟 and 和 often overlap and are interchangeable in “with someone” contexts like:

跟朋友吃饭 / 和朋友吃饭 – eat with friends
跟同事一起开会 / 和同事一起开会 – have a meeting with colleagues

For a learner, you can treat 跟 ≈ 和 when they mean “with” + person, though there are other uses where they behave differently.

Is 上网 literally “go up the internet”? What does it mean exactly, and is it used like a verb?

上网 is a common verb that means:

to go online
to access / use the internet, often with a feel of “surfing/browsing”

You can use 上网 just like a regular verb:

我在家上网。 – I go online at home.
你每天上几个小时网？ – How many hours a day do you spend online?

Grammatically, 上网 is often treated as a verb-object compound (verb 上, object 网), but in practice it behaves a lot like a single verb, and learners usually just memorize it as “to go online / surf the internet”.

Why is there no word like “will” or past tense marker in this sentence? How do we know the time?

Chinese normally does not mark tense (past / present / future) the way English does. Instead, it relies on:

Time words: 明天, 昨天, 现在, etc.
Context
Aspect particles like 了, 过, 在, etc. when needed.

In 她一下课就跟同学一起上网, we just see a habitual pattern:

It can mean “As soon as she finishes class, she (always) goes online with her classmates.”
(a general habit)
In the right context, it could also refer to the future:
“As soon as she finishes class, she will go online with her classmates.”

English must choose a tense (“goes / will go”), but Chinese doesn’t; the context supplies that information.

What does 下课 mean exactly? How is it different from 上课 and 放学?

These three are easy to mix up:

上课
- as a verb: “have class / attend class / teach class”
- e.g. 老师上课了。 – The teacher has started class.
- e.g. 我们八点上课。 – We have class at 8.
下课
- as a verb: “class ends / finish class”
- e.g. 我们五点下课。 – We finish class at five.
- In the sentence: 她一下课就… – As soon as she gets out of class…
放学
- as a verb: “school is over for the day / dismiss for the day” (broader than one class)
- e.g. 三点半放学。 – School ends at 3:30.

So 下课 = the end of a particular class.
放学 = the end of the school day.

The word order feels different from English. Could I say 她跟同学一下课就一起上网?

That word order is not natural. In the 一…就… structure, the 一 + (first event) normally stays together:

Correct patterns:

她一下课就跟同学一起上网。
她下课就跟同学一起上网。 (without 一, still fine)

If you move 跟同学 inside 一下课就, like:

她跟同学一下课就一起上网。

it breaks the rhythm of the 一…就… pattern and sounds wrong to native speakers.

A simple guideline:

Put subject first (她)
Then the 一…就… time clause for the first action (一下课就)
Then the rest (跟同学一起上网)

So: 她 + 一下课就 + 跟同学一起上网。

Could we say 她下课以后就跟同学一起上网 instead? Is that the same as 她一下课就…?

You can say:

她下课以后就跟同学一起上网。

This is correct and natural. The difference is subtle:

一下课就… – emphasizes immediacy: the moment class is over
下课以后就… – “after class, (then) she …”; it can still be soon, but the “as soon as” feeling is a bit weaker.

Also note:

You usually say either 一下课就… or 下课以后就…
不要 say 一下课以后就… – that sounds redundant and unnatural, because 一…就… already has the “as soon as” meaning built in.

Could we use 了 in this sentence, like 她一下课就跟同学一起上网了? What would that change?

Yes, you can add 了 at the end:

她一下课就跟同学一起上网了。

Possible meanings:

Describing a specific past or future event that is now decided/real:
“As soon as she finished class, she went online with her classmates.”
or in context: “As soon as she finishes class, she will (definitely) go online with her classmates.”

Without 了:

她一下课就跟同学一起上网。
Often sounds more like a habit or general fact.

So:

No 了 → more general / habitual reading
With 了 → more event-like, completed or decided, depending on context