Learn Hardcore Chinese: wǒ bú yào shuǐ,wǒ yào yì bēi chá. - I don’t want water; I want a cup of tea.

Because 不 (bù, 4th tone) changes to bú (2nd tone) before another 4th‑tone syllable. 要 is yào (4th), so you get bú yào. Otherwise it stays bù: bù máng (not busy), bù dǒng (don’t understand). Tip: It’s also bú shì (“is not”) since 是 is 4th tone.

Why is 一 pronounced yì in 一杯?

Tone sandhi for 一: it becomes yí before a following 4th tone (e.g., yí gè), and becomes yì before 1st/2nd/3rd tones (e.g., yì tiān, yì běn). Since 杯 is bēi (1st tone), you say yì bēi. When saying the number in isolation or for emphasis, keep yī.

Do I have to repeat 我 in the second clause?

No. You can say 我不要水，要一杯茶。 Repeating 我 is also fine and common, especially in careful speech or writing.

Is 我不要水 too blunt? How can I make it more polite?

It can sound a bit direct. Softer options:

Refusing an offer: 不用水，谢谢。 or 水不用了，谢谢。
Ordering: 请给我一杯茶。 / 麻烦给我一杯茶。 / 我想要一杯茶。
Add a softening particle: 给我一杯茶吧。

What does 要 mean here? Is it “want,” “need,” or “will”?

With a noun, 要 usually means “to want (to have).” So 我要一杯茶 = “I want a cup of tea.” With verbs or time expressions, 要 can mean intention or scheduled future: 明天我要开会 (I’m going to have a meeting). Context disambiguates; here it’s desire, not necessity.

Why do we say 一杯茶 but just 水 with no measure word?

When specifying a quantity, Chinese uses a measure word: 一杯茶, 一瓶水.
When you’re not specifying an amount and just mean “(some) water,” a bare mass noun is fine: 我要水 / 不要水. If you want a specific container, add the measure word: 一杯/一瓶/一壶水.

What’s the difference between 杯 and 杯子?

杯 is a measure word meaning “cupful/glass of.” It appears in quantity phrases: 一杯茶, 两杯咖啡.
杯子 is the noun “cup; mug; glass.” For the object itself you’d say 一个杯子 (“a cup”). Don’t say 一杯子茶 to mean “a cup of tea”; use 一杯茶.

Is 一杯的茶 correct?

Generally, no. In number + measure + noun sequences you don’t insert 的: say 一杯茶. Use 的 if there’s an adjective/description attaching to the noun: 一杯很烫的茶 (“a very hot tea”). Avoid plain 一杯的茶 in everyday speech.

Can I say 茶一杯 or 我要茶一杯?

This inverted order is literary or menu‑style and sounds marked in everyday speech. Use 一杯茶 in normal conversation. Common ordering patterns include 给我一杯茶(吧) and 来一杯茶 (“I’ll have a cup of tea”).

What’s the difference among 不要, 不用, 不需要, and 没要?

不要 + N/VP: don’t want (N) / don’t (do V). With verbs it’s often an imperative: 不要说话 (“Don’t talk”).
不用 + N/VP: no need (for N) / don’t need to (V). Softer refusal: 不用袋子，谢谢 (“No bag, thanks”).
不需要: “do not need”; more formal/emphatic than 不用.
没要: “didn’t want/ask for” (past or negated completion): 我没要水 (“I didn’t ask for water”).

How would I ask “Do you want water or tea?”

Use 还是 for an A‑or‑B question:

你要水还是茶？ Yes/no versions:
你要水吗？
你要不要水？ (A‑not‑A pattern)

Should I add 喝? What’s the difference between 我要一杯茶 and 我要喝茶?

我要一杯茶 focuses on the item/quantity (“a cup of tea”).
我要喝茶 focuses on the action (“I want to drink tea”) and doesn’t specify amount. Both are natural; in a café, the first is more precise.

Any tone‑sandhi issues besides 不 and 一 here?

The main mandatory ones are the 不 → bú and 一 → yì changes already shown. 我 (wǒ, 3rd) is followed by bú (2nd), so it stays 3rd. After a pause (the comma), 水 (shuǐ, 3rd) is followed by 我 (wǒ, 3rd) but the pause resets sandhi, so you pronounce each as 3rd. In fast speech, 3rd tones often sound like a low “half‑third,” but you don’t change them to 2nd here.

Is the comma necessary? Could I use 但是?

In Chinese, a comma often links two short clauses with a natural pause: 我不要水，我要一杯茶。 You may add a contrastive connector for emphasis: 但是/可是我不要水，我要一杯茶。 Both are fine; the version without a connector is very common.

Can I drop 一 and say 我要杯茶?

In casual speech, yes: 我要杯茶 is commonly heard, just like 要个 for 要一个. In careful speech and writing, keeping 一 is standard: 我要一杯茶.

How do I say “two/three cups of tea,” or “a bottle/a pot of” something?

Use number + measure + noun:

两杯茶 (liǎng bēi chá) two cups of tea
三杯茶 (sān bēi chá) three cups of tea
Common containers: 一瓶水 (a bottle of water), 一壶茶 (a pot of tea), 一杯咖啡 (a cup of coffee)

How do I say “I don’t want water anymore; I’ll have tea instead”?

Use change‑of‑state 了: 我不要水了，我要一杯茶。 It implies your preference has just changed.

What’s the nuance between 要, 想要, and 想喝?

要 + N: straightforward want/request; more direct.
想要 + N: “would like to have”; softer and more tentative.
想 + V/ 想喝: “feel like (doing/drinking)”; focuses on the action. In service settings, any is fine; add polite phrasing if you want to sound gentler.

Is 我要茶 acceptable, without 一杯?

Grammatically yes: 我要茶 means “I want tea (rather than water).” But in a café/restaurant you normally specify the unit: 我要一杯茶 or 我要一壶茶. Bare 茶 sounds general or abstract.