Learn Hardcore Chinese: wǒ qù gōngyuán sànbù de shíhou, shùnbiàn mǎi yì bēi kāfēi. - When I go to the park for a walk, I also buy a cup of coffee while I’m at it.

You can also say 我去公园散步时, dropping 的 and using 时 instead of 时候. That’s a bit more formal/literary. In everyday speech, 的时候 is much more common and sounds natural.

Can I omit 的 and just say 我去公园散步时候?

No, that sounds wrong in modern standard Mandarin.

For this pattern in normal spoken and written Chinese, you should use:

我……的时候
or the more formal:
我……时

So:

✔ 我去公园散步的时候
✔ 我去公园散步时 (formal)
✘ 我去公园散步时候 (unnatural)

Why is the part 我去公园散步的时候 put at the beginning of the sentence?

Chinese often puts time or condition clauses at the beginning, before the main action:

我去公园散步的时候，顺便买一杯咖啡。
→ “When I go to the park for a walk, I (will) also buy a cup of coffee.”

Pattern:

[Time/condition clause]，[main action].

Other examples:

下雨的时候，我在家看书。 – When it rains, I read at home.
你有空的时候，给我打电话。 – When you’re free, give me a call.

You can sometimes move the time phrase, but with a longer clause like 我去公园散步的时候, putting it first is the most natural and clear.

Why isn’t there a second 我 before 顺便买一杯咖啡?

Chinese often drops repeated subjects when it’s clear who is doing the action.

Full, explicit version:

我去公园散步的时候，我顺便买一杯咖啡。

Since both actions are done by 我, the second 我 is normally omitted:

我去公园散步的时候，顺便买一杯咖啡。

English usually repeats “I”:

“When I go to the park for a walk, I’ll also buy a coffee.” but Chinese doesn’t need to. As long as the subject is obvious from context, it can be left out.

What exactly does 顺便 mean? Is it just “then” or “also”?

顺便 (shùnbiàn) means “while you’re at it / incidentally / as a side thing because it’s convenient.”

It implies:

You already plan to do Action A.
You do Action B at the same time or on the way, because it’s convenient, not because B is the main goal.

In this sentence:

Main purpose: 去公园散步 (go to the park for a walk)
Extra convenient action: 顺便买一杯咖啡 (buy a coffee while you’re at it)

Compare:

然后 (ránhòu) – “then / and then” (just a sequence in time)
也 (yě) – “also” (adds another action, but no “convenience” nuance)

Examples:

你去超市的时候，顺便帮我买点牛奶。
When you go to the supermarket, could you also (while you’re there) buy me some milk?
我明天去公司，顺便把文件给你。
I’ll be going to the office tomorrow; I’ll give you the documents while I’m there.

Why is it 去公园散步 instead of 在公园散步?

Both are possible, but they highlight slightly different things:

去公园散步
- Literally: “go to the park to take a walk”
- Emphasizes the going (去) as part of the event. It’s like including the trip as part of your plan.
在公园散步
- Literally: “take a walk in the park”
- Emphasizes the activity at that location, not the act of going there.

Your sentence:

我去公园散步的时候，顺便买一杯咖啡。
→ When I go to the park (to take a walk), I’ll also buy a coffee.

If you say:

我在公园散步的时候，顺便买一杯咖啡。
it sounds more like you buy the coffee while you are already walking in the park. That is possible (for example, if there’s a café inside the park), but the original suggests more generally: on the way there / as part of that outing.

What’s the difference between 散步, 走, and 走路 here?

All involve walking, but with different typical uses:

散步 (sànbù) – “to take a walk / stroll (for leisure, exercise, relaxation).”
- Often used with going out just to walk.
- 去公园散步 = go to the park for a walk (as an activity).
走 (zǒu) – basic verb “to walk; to leave; to go.”
- 我走路去公司。 – I walk to work.
- 我先走了。 – I’m leaving now.
走路 (zǒu lù) – literally “walk (on the road)”; just the basic act of walking, not necessarily for pleasure.
- 我每天走路上班。 – I walk to work every day.

So:

我去公园散步 = I go to the park to take a stroll (for pleasure/exercise), which matches the idea of going out to relax.
我去公园走路 is understandable but not idiomatic; Chinese speakers naturally say 散步 in this “take a walk” context.

Why do we need 杯 with 咖啡? Can’t we just say 买一咖啡?

In Chinese, you almost always need a measure word between a number and a noun.

Pattern:

Number + Measure word + Noun

Here:

一杯咖啡
- 一 – one
- 杯 – cup (measure word)
- 咖啡 – coffee
  → “a cup of coffee”

You can’t say:

✘ 买一咖啡 (ungrammatical in standard Mandarin)

Other examples:

一个人 – one person
三本书 – three books
两辆车 – two vehicles/cars
四杯茶 – four cups of tea

For drinks in cups/glasses, 杯 is the standard measure word.

Why is 一 pronounced yì in 一杯咖啡, not yī or yí?

This is due to tone sandhi (tone change rules) for 一 (yī).

Rules for 一:

Before a 4th-tone syllable, 一 is pronounced yì (4th tone).
Before a 1st, 2nd, or 3rd tone syllable, 一 is usually pronounced yí (2nd tone).
When said alone or stressed, it’s yī (1st tone).

In 一杯咖啡:

杯 (bēi) is 1st tone.
Before a 1st tone, 一 changes to yí in many textbooks, but in actual speech, a lot of speakers use yì before measure words as a kind of fixed pattern: yì bēi, yì běn, yì gè, etc. Both analyses exist; what matters is that you’ll hear yì bēi.

More clear examples:

一个人 → yí ge rén (个 is neutral, often treated like 4th tone in teaching, hence yì ge rén is also common in real speech)
一条鱼 (yú, 2nd tone) → yí tiáo yú
一块蛋糕 (kuài, 4th tone) → yì kuài dàngāo

For learners, it’s fine (and very common) to say:

yì bēi kāfēi for 一杯咖啡.

Should there be a 了 after 买 to show past tense, like 买了一杯咖啡?

Not necessarily. Chinese doesn’t mark tense (past/present/future) the same way English does. It often relies on context and other time words.

我去公园散步的时候，顺便买一杯咖啡。
Can mean:
- “When I go to the park for a walk, I (usually) also buy a cup of coffee.” (habitual)
- “When I go to the park for a walk, I will also buy a cup of coffee.” (future plan)

If you add 了:

我去公园散步的时候，顺便买了一杯咖啡。
This sounds more like a specific past event:
- “When I went to the park for a walk, I (then) bought a cup of coffee.”

So:

Without 了: more neutral, can be habitual or future depending on context.
With 了: more clearly refers to a completed action in a specific instance.

Can I drop 一 and just say 买杯咖啡? What’s the difference?

Yes, you can say:

顺便买杯咖啡。

In spoken Chinese, 一 is often dropped before a measure word when it’s not being emphasized.

Difference in feel:

买一杯咖啡 – slightly more explicit: “buy one cup of coffee.”
买杯咖啡 – very natural and casual: “buy a coffee.”

It’s similar to English:

“I’m going to buy a coffee.”
“I’m going to buy one coffee.” (a bit more explicit/emphatic)

Both forms are correct; 买杯咖啡 is extremely common in everyday speech.

Could I start the sentence without 我, like 去公园散步的时候，顺便买一杯咖啡。?

Yes, you can drop the first 我 in the right context:

去公园散步的时候，顺便买一杯咖啡。

This sounds like a reminder or instruction to you/someone specific, where the subject is understood:

“(When you) go to the park for a walk, (you should) also buy a coffee.”

In isolation, 我去公园散步的时候，顺便买一杯咖啡。 is clearer about who is doing it (I).
If you’re talking about your own habit/plan and want to be explicit, keep 我. If you’re giving an instruction or suggestion, or the subject is already established in the previous sentence, you can drop it.