Learn Hardcore Chinese: tā xiǎng zìjǐ qù chāoshì. - She wants to go to the supermarket by herself.

With a verb phrase, 想 + V means want to / would like to / plan to: 她想自己去超市 = She wants/plans to go by herself.
With a clause, 想 can mean to consider/plan: 她想明天去 = She’s thinking of going tomorrow.
For opinions, use 觉得/认为: 她觉得一个人去更方便 = She thinks it’s more convenient to go alone.
With a noun/pronoun, 想 can mean miss: 我想你 = I miss you.

What exactly does 自己 (zìjǐ) do here? Who does it refer to?

自己 is a reflexive pronoun meaning oneself; it refers to the subject of the relevant clause.
In 她想[自己去超市], 自己 stands for the understood subject of the inner clause and refers back to 她. So: she wants to go by herself.
Compare:
- 妈妈想她自己去超市 = Mom wants her to go by herself.
- 妈妈想自己去超市 = Mom wants to go by herself.

What’s the difference between 她想自己去超市 and 她自己想去超市?

她想自己去超市: focus on doing the action alone (by herself).
她自己想去超市: focus on who has the desire (she herself wants to go, not someone else). It doesn’t necessarily mean she’ll go alone.

Can I say 一个人 instead of 自己?

Yes: 她想一个人去超市 ≈ she wants to go alone.
Nuance:
- 自己 emphasizes doing it by oneself (no help).
- 一个人 emphasizes no companion.
Both are often interchangeable; you can also say 自己一个人 or 独自 for extra clarity/emphasis.

Why is there no preposition like “to” before 超市? Is 去 enough?

Chinese uses 去 + place for motion: 去超市 = go to the supermarket. No preposition needed.
You can also say 到超市去 (slightly more formal/emphatic on arrival).
去 emphasizes the going process; 到 emphasizes reaching the destination.

Where would 了 go if I want to talk about already going, or a change of desire?

Already went: 她已经自己去超市了.
Change of desire (now she wants to): 她想去超市了 / 她想自己去超市了 (implies a new development).
Don’t use 了 just to mark simple present/future desire.

How do I negate this? Is it 不想 or 没想?

Desire negation: 不想 = do not want to. Example: 她不想自己去超市.
没想 = didn’t think/plan/expect (past or factual), not a simple “doesn’t want.” Example: 她没想自己去超市 = She hadn’t planned to go by herself.

How can I turn it into a question?

Yes/no with 吗: 她想自己去超市吗？
A-not-A for the verb 想: 她想不想自己去超市？
Asking about the action choice: 她想自己去不去超市？ (= she’s considering whether to go by herself).

Is 去 required? Could I say 她想超市?

You need a motion verb; 她想超市 is ungrammatical.
Use 她想去超市 (optionally with 自己). In casual ellipsis you can drop words only when context makes the verb obvious, but with 想 you normally keep the verb.

Pronunciation and tones for each word?

Do I need a measure word with 超市 here?

After 去, place nouns don’t take a measure word: 去超市 is standard.
Use a classifier when counting/specifying: 一家超市 = a supermarket. Example: 她想去一家超市 (go to a particular/unspecified one).

How do I say “make a quick trip to the supermarket”?

What’s the difference between 想, 要, and 想要 here?

想 + V: would like to / intends to (politer, softer).
要 + V: stronger intention, plan, or necessity; can sound more determined/insistent. 她要自己去超市 = She will/insists on going by herself.
想要 + V/N: want (often stronger or more deliberate). 她想要去超市 is fine but a bit heavier than 想去; 想要 + N is very common (e.g., 想要苹果).

Where do time words or adverbs go?

Modify the wanting: 她现在想自己去超市 = Now she wants to go by herself.
Modify the going time: 她想现在就自己去超市 = She wants to go right now, by herself.
Modals like 可能 go before the VP: 她可能自己去超市. If it’s her thought, use a full clause: 她想她可能会自己去超市.

Could this mean “She thinks she will go by herself,” not just “wants”?

Yes, 想 can also mean “is thinking of/plan to,” so 她想自己去超市 can be read as “She’s thinking of going by herself.”
For an opinion-type “think,” prefer 觉得/认为: 她觉得自己去超市更方便.

If I add another action like 买东西, where does 自己 go?

Default and most natural: put 自己 before the first verb of the action chain: 她想自己去超市买东西.
她想去超市自己买东西 is also possible, shifting the focus to “she herself will do the buying,” but the first version is the safest general pattern.