Learn Hardcore Chinese: wǒ mǎi le shǒujī, tā yě xiǎng mǎi. - I bought a phone; she also wants to buy one.

The 了 after 买 is the perfective aspect marker. It marks the action as completed: 我买了手机 = the buying is done. It’s not a general “past tense” marker—Chinese doesn’t have tense; time is usually given by context or time words (like 昨天).

Could I use 过 instead of 了?

No. 过 marks past experience (“have ever done”): 我买过手机 = I have bought a phone before (at some point in my life). 了 marks a specific completed event: 我买了手机 = I bought a phone (this time).

Do I need a measure word with 手机?

Only when you specify a number or “one.” Then use a measure word: 一部手机 (most idiomatic for phones), 一台手机 (also common), or the general 一个手机 (colloquial). Without a number, 买了手机 is fine and means “bought a phone (unspecified).”

Why is there no object in the second clause “她也想买”?

Chinese often omits repeated information. The object 手机 is understood from the first clause. You can include it for clarity: 她也想买（一个）手机 or just 她也想买一个.

Where should 也 go?

Place 也 after the subject and before the verb/adjective: 她也想买. Don’t say 她想也买 or 也她想买. With negation, it’s still before the verb: 她也不想买 / 她也没买.

Why is there just a comma instead of 和/而且 to connect the clauses?

Chinese often links two related clauses simply with a comma. 和 is mainly for linking nouns, not whole clauses. 而且 can be used in “not only…but also” or to add a stronger second point, but here the simple comma (with 也) is the most natural.

Does 想 mean “to think” or “to want” here?

Here 想 + verb means “to want to”: 她也想买 = she also wants to buy. Other meanings of 想 include “to think” (我想她会来) and “to miss” (我想你), but not in this sentence.

What’s the difference between 想买, 要买, and 想要买?

想买: would like to buy (softer desire).
要买: (intend to) buy; more decided or immediate.
想要买: emphasizes desire; a bit stronger than 想买. In many contexts, 想买 or 要买 is enough.

How do I negate each part correctly?

Completed action: 我没(有)买手机 (“I didn’t buy a phone”). Don’t use 不 here.
Desire: 她也不想买 (“She also doesn’t want to buy”).
Note: 不买了 means “I’ve changed my mind; I won’t buy (anymore).” 她也没想买 = “She also didn’t (ever) intend/want to buy” (different nuance).

Can I say 我买手机了? How is that different from 我买了手机?

我买了手机: neutral report of a completed event (“I bought a phone”).
我买手机了: sentence-final 了 highlights a new situation/change of state (“I’ve bought a phone now”). Both can describe completion, but the second feels more like an update.

Why not say 她也想买了?

了 marks completion of an action; 想 (“to want”) is a mental state and doesn’t take 了 this way. If she also bought one, say 她也买了（手机/一部手机）.

Can I drop the subject in the second clause?

Yes, if it’s clear from context. You could say 也想买 or 也想买一个 in conversation. In writing or when clarity matters, keep 她.

Any tone-sandhi tips for pronouncing this?

Third-tone sandhi applies: a third tone becomes second tone before another third tone. So you’ll naturally say:

我买 as “wó mǎi”
也想 as “yé xiǎng”
想买 as “xiáng mǎi” The particle 了 is neutral tone. 手机 is “shǒu-jī” (3-1).

Should I use 也 or 都 if both people want to buy?

Use 都 with a plural subject: 我和她都想买手机 (“We both/all want to buy a phone”). Use 也 to add another subject to an existing statement: 我想买手机，她也想买 (“I want to buy a phone; she also wants to”).