Tā yí xiàkè jiù gēn tóngxué yìqǐ shàngwǎng.

In this sentence, 一 is not the number “one”. It is part of the fixed pattern:

一 + event 1 + 就 + event 2

This pattern means “as soon as … then …” or “no sooner … than …”.

So 一下课就 means:

• 一 (yí) – “as soon as”
• 下课 – finish class
• 就 – then / immediately

Together: 她一下课就… = “As soon as she finishes class, she …”

Here are a couple more examples of the same pattern:

• 我一到家就给你打电话。
  As soon as I get home, I’ll call you.

• 他一看到狗就很高兴。
  As soon as he sees a dog, he’s very happy.

So think of 一…就… as a grammar pattern, not as the number “one” here.

In 她一下课就…, you should not split it as 一 + 下 + 课 in the usual “do something + 一下” sense.

There are two different things that look similar:

1. Pattern in this sentence:

   • 一 + 下课 + 就…
   • 下课 is the whole verb “to finish class”
   • 一…就… = “as soon as … then …”

2. 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.

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.

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.

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.

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.

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”.

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.

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.

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。

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.

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