Learn Hardcore Chinese: wǒmen yǐjīng mǎi le shǒujī.

It’s the perfective aspect particle marking a completed action. In 我们已经买了手机, 了 shows the buying event is finished. It is not a general past-tense marker; it specifically highlights completion.

Do I need both 已经 and 了? Aren’t they redundant?

They’re not redundant. 已经 means “already” (temporal meaning), while 了 marks completion. With dynamic verbs like 买, you typically use 已经 + 了 to say an action has already been completed: 我们已经买了手机. If the predicate is stative, you may only need sentence-final 了: 我已经饿了. You can also use sentence-final 了 instead of verb-了: 我们已经买手机了.

How do I negate this meaning (“We haven’t bought a phone yet”)?

Use 还没(有) and do not use 了:

我们还没(有)买手机(呢)。 Using 没(有) with 了 is ungrammatical: ✗我们没买了手机.

Where does 已经 go in the sentence?

Before the verb phrase, usually right after the subject: 我们已经买了手机. With other adverbs, the common order is subject + 都/也 + 已经 + verb phrase:

我们都已经买了手机。
我们也已经买了手机。

Can I say 我们已经买手机了 instead of 我们已经买了手机?

Yes. Both are natural:

我们已经买了手机: neutral “completed action.”
我们已经买手机了: also completed, but with a stronger “now it’s the case that…”/change-of-state feel (because 了 is sentence-final). In everyday speech both are fine.

What’s the difference between 买了 and 买过?

买了: a specific completed event (we completed the purchase).
买过: experiential; you have had the experience before, not necessarily referring to a specific, recent purchase.
Examples:
我们已经买了手机。 (We completed the purchase.)
我们已经买过手机了。 (We have bought phones before; experience exists.) 过 usually doesn’t pair with a specific time like 昨天.

Can I add a second 了 at the end: 我们已经买了手机了?

Yes, in colloquial speech you’ll hear both a verb-了 and a sentence-final 了 together. The second 了 adds a “new situation/change-of-state” or an emphatic tone. In careful writing, many prefer to avoid the “double 了” unless that nuance is intended.

Why is there no measure word before 手机? Shouldn’t it be 一个/一部手机?

You need a measure word when you use a numeral or a demonstrative. Here, 手机 is an indefinite object with unspecified quantity, so a bare noun is fine. If you want to specify quantity, add it:

买了一部手机 (one phone)
买了两部手机 (two phones)

Which measure word should I use for 手机?

Most common/specific: 部 (一部手机)
Generic and very common in speech: 个 (一个手机)
Another option for machines: 台 (一台手机)
All are understandable; 部 is the safest “correct” choice.

Is it one phone or several? The Chinese looks ambiguous.

It’s intentionally unspecified. It could be one shared phone or multiple phones for different people. To disambiguate:

One: 我们已经买了一部手机。
Each person got one: 我们每人已经买了一部手机。/ 我们各买了一部手机。
Several (unspecified): 我们已经买了几部手机。/ 我们已经买了一些手机。

How do I form the yes/no question for this?

Use 了吗:

你们买手机了吗？ Answer:
Affirmative: 我们已经买了手机。
Negative: 我们还没(有)买手机。

Can I drop the subject 我们?

Yes, if it’s clear from context. For example, as an answer:

Q: 你们买手机了吗？
A: 已经买了。

Where do time words like 昨天 go?

Typically after the subject and before 已经, or sentence-initial:

我们昨天已经买了手机。
昨天我们已经买了手机。 Avoid placing 昨天 after 已经: ✗我们已经昨天买了手机.

What’s the difference between 买了手机 and 有手机了?

买了手机 focuses on the action of buying being completed.
有手机了 focuses on the resulting state (now we have a phone), with sentence-final 了 marking a new situation:
- 我们已经有手机了。 (We now have a phone; new state established.)

How can I emphasize when/where/how we bought it?

Use the 是…的 construction to spotlight circumstances of a past event:

我们是昨天在商场买的手机。 (It was yesterday at the mall that we bought the phone.) This construction doesn’t need 已经/了, because it presupposes the action happened.

Can 已经 go at the end or after the object?

No. 已经 should come before the predicate it modifies. Correct placements:

我们已经买了手机。
手机我们已经买了。 (Topicalized object) But not: ✗我们买了手机已经。

What’s the difference between 买到了手机 and 买了手机?

买到了手机 uses the result complement 到 to highlight successful attainment (“managed to get”). Good when availability was uncertain.
买了手机 simply states completion of buying, without emphasizing difficulty/success.

How do I pronounce 了 here? Is it le or liǎo?

Here it’s the aspect particle pronounced neutral-tone le. The liǎo reading appears in other words (e.g., 了解 liǎojiě), not in this sentence.

Any tone sandhi to watch out for?

Third tone is often realized as a low “half-third” before non–third tones. In wǒmen yǐjīng mǎi le shǒujī, none of the adjacent syllables trigger the 3→2 change (there’s no immediate 3rd-tone–3rd-tone pair), but you’ll still naturally compress the third tones.

How do I say “We won’t buy a phone anymore” vs “We haven’t bought a phone yet”?

“No longer (change of plan/habit)”: 我们不买手机了。
“Not yet (but maybe later)”: 我们还没(有)买手机。

Can I use 把 here?

Yes, if the object is definite/known:

我们已经把手机买了。 (That specific phone we discussed is already bought.) Often you’ll see a result complement with 把 to stress completion: 我们已经把手机买好了。 (It’s bought and all set.) Don’t use 把 with an indefinite, unknown phone.

Is there a stronger way than 已经 to say this?

Use 早就 for “long ago/already for a while”:

我们早就买了手机。 (We bought a phone long ago.)