Learn Hardcore Chinese: xiàkè yǐhòu wǒmen yìqǐ qù gōngyuán sànbù. - After class we will go take a walk in the park together.

Putting 以后 before the event (以后下课) sounds unnatural or confusing; it would be interpreted more like “in the future, class ends…”, which is not what you want here.

So 下课以后我们一起去公园散步 is natural; 以后下课我们一起去公园散步 is not.

What is the difference between 下课以后 and 下课的时候?

Both are time expressions but with different nuances:

下课以后 – after class (is over)
- Focus: after the ending point.
- Implies the action happens sometime later, not at the exact moment of ending.
下课的时候 – when it is class-over time / when class ends
- Focus: the time around when class ends, more centered on that point.

In your sentence, 下课以后我们一起去公园散步 emphasizes that after class is finished, at some point, you’ll go for a walk.
You could say 下课的时候，我们一起去公园散步, but it sounds more like “at the time of class ending, we (already) go to the park for a walk”, slightly less natural for a planned future activity than 以后.

Why is there no word like “will” in this Chinese sentence if it’s talking about the future?

Chinese usually does not mark tense the way English does. Future time is indicated by:

Time expressions: 下课以后 (after class), 明天 (tomorrow), etc.
Sometimes modal verbs / adverbs: 会 (will), 要 (be going to), 打算 (plan to).

In 下课以后我们一起去公园散步, the combination of 下课以后 and the context is enough to tell us it’s about the future, so you don’t need 会 or 要.

You could say:

下课以后我们会一起去公园散步。
下课以后我们要一起去公园散步。

These add a slight nuance of intention / prediction, but the original is already perfectly natural.

Where is the equivalent of English “to” in “go to the park”? Why not 去到公园 here?

In this sentence, 去 itself already means “go (to)”:

去公园 = go to the park
去北京 = go to Beijing

You only add 到 in certain structures, often for emphasis on arrival:

到公园去散步 – (go to the park and arrive there to walk)
我到了公园才给你打电话。 – I only called you after I arrived at the park.

For a simple “go to the park (and do something)”, 去公园 is the normal, simpler form. So 下课以后我们一起去公园散步 is exactly like “After class we (will) go to the park for a walk.”

Why is 一起 placed before 去公园散步? Can I move it to another position?

一起 is an adverb meaning “together”, and adverbs usually go before the main verb phrase:

我们一起去公园散步 – We together go to the park for a walk.

Other common adverb positions:

我们都常常一起去公园散步。
(subject) + (adverbs: 都, 常常) + 一起
- verb phrase

Possible variations:

下课以后，我们一起去公园散步。 ✅
下课以后，我们去公园一起散步。 😐 (understandable but less natural; 一起 much more often comes before the first main verb 去, not in the middle of 去公园散步)

Best to remember: subject + (time) + 一起 + verb phrase is the normal pattern.

Why is there no 在 before 公园? Shouldn’t it be 在公园散步?

There are actually two common patterns:

去 + place + action
- 我们去公园散步。 – We go to the park (and) take a walk.
  Here, 去公园 is movement (go to the park), and 散步 is what you do there.
在 + place + action
- 我们在公园散步。 – We take a walk in the park.
  Here, 在公园 sets the location, no movement verb.

Your sentence uses pattern 1: 下课以后我们一起去公园散步 emphasizes the going (to the park) and then walking there.
If you say:

下课以后我们一起在公园散步。

it sounds like you’re focusing less on the going, more on the fact that the walk happens in the park. Both are grammatical; the original is just slightly more “let’s go there and walk.”

Why do we need both 去 and 散步? Could I just say 下课以后我们一起去公园?

去公园 = go to the park (but doesn’t say what you do there)
散步 = take a walk / go for a walk

下课以后我们一起去公园散步 gives both:

destination: 公园
activity: 散步

If you only say 下课以后我们一起去公园, it means “After class, we’ll go to the park (for some purpose)”, but you’re not specifying that the purpose is walking. Context might make it clear, but the sentence itself is less specific.

Is 散步 a verb, a noun, or a verb–object phrase? Can I split it?

散步 is usually treated as a verb-object structure: 散 (to stroll) + 步 (steps). Functionally it behaves very much like a single verb meaning “to take a walk”.

You can:

Use it as a whole verb:
- 我们去公园散步。 – We go to the park to take a walk.
Insert some elements inside:
- 散一会儿步 – take a walk for a while
- 散散步 – take a little walk / stroll

But you cannot separate it arbitrarily:

✗ 散公园步 – incorrect
✗ 散步公园 – incorrect

So for your sentence, 去公园散步 is the natural, fixed order.

Can I drop 我们 and just say 下课以后一起去公园散步?

Yes, in many contexts Chinese omits the subject when it’s obvious from context.

Among friends making plans, if everyone knows you’re talking about “us,”
下课以后一起去公园散步吧。
is perfectly natural.

However, with no context, 下课以后我们一起去公园散步 is clearer, especially in a textbook/example sentence. Omitting 我们 is more common in spoken, context-rich situations.

Can I add 了 somewhere, like 下课以后我们一起去公园散步了?

Adding 了 changes the meaning:

(future plan / suggestion)
- 下课以后我们一起去公园散步。 – After class let’s go / we’ll go for a walk in the park.
(description of a completed event)
- 下课以后我们一起去公园散步了。 – After class, we (indeed) went to the park for a walk.

Sentence-final 了 here usually indicates a change of situation or completed event.
So:

For planning / suggesting → no 了.
For reporting what actually happened → use 了 is natural.

How exactly is 一起 pronounced, and is there any tone change?

一起 is pronounced:

一: normally yī (first tone), but before a fourth-tone syllable it changes to second tone: yí.
起: qǐ (third tone).

In 一起, because 起 is a third tone, 一 keeps its first tone, so it’s:

yìqǐ → written as first + third, but in actual speech 一 often sounds like a high, clipped tone (close to yì).

Tones:

一 (in this word): yì (high, short)
起: qǐ

So: 一起 → yìqǐ.