Learn Hardcore Chinese: wǒ zài zhè gè chéngshì zhù le sìshí tiān le. - I have lived in this city for forty days.

The first 了 (after 住)
- This is verb 了 / aspect 了.
- It attaches to the verb 住 and works with the duration 四十天 to show that the action has already lasted that long.
- Pattern: V + 了 + Duration (+ 了)
The second 了 (at the end)
- This is sentence-final 了, which marks a new situation / change of state.
- It roughly adds the idea of “now” or “already (by now)”:
  - Now it has been 40 days (that I’ve been living here).

Together, 住了四十天了 means:

The action started in the past, has continued for 40 days, and the speaker is still living there now.

What is the difference between 我在这个城市住了四十天了 and 我在这个城市住了四十天 (without the last 了)?

我在这个城市住了四十天了
- Implies: I have (already) been living in this city for 40 days, and I’m still here now.
- Ongoing situation up to the present.
我在这个城市住了四十天
- Normally understood as: I lived in this city for 40 days (and then I left / I’m not living there now).
- Describes a completed period in the past.

So:

With final 了 → ongoing up to now.
Without final 了 → finished in the past (unless context forces a different reading).

Could I say 我在这个城市住四十天了 (without the first 了)?

That sounds unnatural in standard Mandarin.

For this meaning (“I have been living…for 40 days (already)”), Mandarin normally uses:

V + 了 + Duration + 了
→ 住了四十天了

If you drop the first 了, it breaks this common pattern for expressing “have been doing X for Y time”.

Natural options:

我在这个城市住了四十天了。
You can add 已经 for emphasis: 我已经在这个城市住了四十天了。

What exactly does 在 (zài) do here? Is it like “am doing” or like “at/in”?

In this sentence, 在 is a location preposition, meaning “at / in”:

我在这个城市住了四十天了。
→ 在这个城市 = in this city (location)
→ 住 = live, reside

So the structure is:
[Subject] + 在 + [Place] + [Verb] ...

This 在 is not the same use as in:

我在吃饭。 (I am eating.)

There, 在 works more like a progressive marker (“be doing”).
Here, it’s simply “at / in this city”.

Can I say 我住在这个城市四十天了 instead? What’s the difference in word order?

Yes, 我住在这个城市四十天了 is understandable, but for this specific durative pattern, the more standard / natural form is:

我在这个城市住了四十天了。

Compare structures:

我在这个城市住了四十天了。
- 在这个城市 is a location phrase before the verb.
- Very common and feels natural.
我住在这个城市。 (without duration)
- 住在 + Place = “live in X” as one chunk.
- Perfectly natural for simple statements of residence.

When you add a duration and 了…了, Mandarin strongly prefers:

Subject + 在 + Location + 住 + 了 + Duration + 了

So:

For “I live in this city” → 我住在这个城市。
For “I have been living in this city for 40 days” → 我在这个城市住了四十天了。

What is the function of 个 (gè) in 这个城市? Could I use another measure word?

个 is the measure word / classifier for 这:

这 + 个 + 城市 → this (one) city

Key points:

个 is the default, most common classifier, and it’s fine with 城市.
Another, slightly more “formal / literary” or “descriptive” classifier is 座 (zuò), often for buildings, bridges, cities, mountains:
- 这座城市 = this city (a bit more descriptive / written-style)

So you can say:

我在这个城市住了四十天了。 (very normal)
我在这座城市住了四十天了。 (also correct, slightly more literary / descriptive)

You do not say 这城市 in standard style when you’re counting; you normally need a classifier: 这个城市 / 这座城市.

Why is 天 (tiān) there? Can I say 四十日 or 四十个天?

天 is the usual word for “day” in everyday speech.
- 四十天 = 40 days

About your alternatives:

四十日
- 日 also means “day”, but it’s more formal / written / literary.
- You might see it in documents, headlines, classical-style writing.
四十个天
- This is wrong. 天 itself is the measure word for days.
- You don’t add 个:
  - ✅ 三天、十天、四十天
  - ❌ 三个天、十个天、四十个天

So in spoken Mandarin, 四十天 is the natural choice here.

What’s the difference between using 了 (le) and 过 (guo) with 住? Could I say 我在这个城市住过四十天?

了 and 过 mark different aspects:

了 (le) here:
- With a duration and final 了, it indicates:
  - The action has lasted for that long
  - And (with final 了) it’s still true now.
- 我在这个城市住了四十天了。 → I have been living in this city for 40 days (and I’m still living here).
过 (guo):
- Marks experiential aspect: you have had that experience at least once in the past.
- 我在这个城市住过四十天。 → I have (at some point) lived in this city for 40 days.
  → You are not saying you’re living here now; it’s a past experience.

So:

住了四十天了 → ongoing situation, present-relevant.
住过四十天 → past experience, no claim about the present.

Can I add 已经 (yǐjīng, “already”)? Where does it go?

Yes, 已经 is very natural here, and it adds emphasis to the “already” meaning.

Common placements:

我已经在这个城市住了四十天了。
我在这个城市已经住了四十天了。

Both are fine. General pattern:

已经 usually appears before the verb or before the duration phrase.

You almost never put it after the duration in this type of sentence:

❌ 我在这个城市住了四十天已经了。 (unnatural)

What’s the difference between 住, 待, and 呆 here? Could I use them instead of 住?

All three can relate to “staying somewhere”, but with different nuances:

住 (zhù)
- Main meaning: live / reside (for some period).
- Neutral about how temporary or long-term it is.
- 我在这个城市住了四十天了。 → I’ve been living here for 40 days.
待 (dāi)
- Means “stay, remain (in a place)”.
- Often sounds a bit more temporary than 住, like staying, hanging around:
  - 我在这个城市待了四十天。
    → I stayed in this city for 40 days.
- You wouldn’t usually use the double 了 pattern here to mean “still staying now”; it often describes a completed stay.
呆 (dāi)
- Also “stay”, with similar usage to 待 in this sense.
- In some regions, 呆 is more common in speech; 待 can feel a bit more formal/written.
- 在这儿呆了几天 = stayed here for a few days.

For “I have been living in this city for 40 days (and I’m still living here)”, 住 with the V了Duration了 pattern is the most natural:

我在这个城市住了四十天了。

How would I say the negative, like “I haven’t been in this city for 40 days yet”?

You don’t use 了 in the negative here. Instead you normally say:

我在这个城市还没住四十天。
→ Literally: I in this city still haven’t lived (for) 40 days.
→ Natural English: I haven’t been living in this city for 40 days yet.

Key points:

Use 还没 / 还没有 + Verb + Duration.
No 了 in the negative form:
- ❌ 我在这个城市还没住了四十天了。 (wrong)
- ✅ 我在这个城市还没住四十天。