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

The first 了 after 住 (住了) is the verb‑particle 了: it marks that the action has reached a certain point (here: the living has already lasted 30 days).
The last 了 at the end of the sentence is sentence‑final 了: it shows a change of situation / a new state (now it has already been 30 days).

Together, they give the sense of English “I’ve already been living in this city for 30 days (up to now).”

Does this sentence imply that I am still living in this city now?

Yes. 我在这个城市住了三十天了 normally means:

You started living in this city 30 days ago,
You are still living there now, and
You’re emphasizing the length so far.

If you want to say you no longer live there, you usually drop the final 了: 我在这个城市住了三十天 (“I lived in this city for 30 days [and then stopped]”). Context still matters, but this is the common contrast.

Can I use only one 了? What happens if I remove one of them?

Common patterns:

我在这个城市住了三十天了 – ongoing up to now (have lived for 30 days and still living).
我在这个城市住了三十天 – completed in the past (lived there for 30 days, but not now).
我在这个城市住三十天了 – generally unnatural; you normally need 了 right after the verb for this duration pattern.

So:

To talk about up‑to‑now ongoing time: use 了…了.
To talk about a finished stay: usually only the verb‑particle 了: 住了三十天 (no final 了).

Is 了 a past tense marker here?

Not exactly. Mandarin doesn’t have real tense markers like English.

The 了 after 住 marks aspect: the action has reached or passed a certain point (here, 30 days).
The sentence‑final 了 marks a new situation: “Now it has already been 30 days.”

So the sentence corresponds to English present perfect (“have lived for 30 days”), but grammatically it’s aspect + new‑situation, not a strict past tense.

Why is the place phrase written as 我在这个城市住… instead of something like 我住在这个城市…?

Both patterns exist, but they’re used slightly differently:

我在这个城市住了三十天了 – 在 + place comes before the verb; very natural with a duration phrase.
我住在这个城市 – 住在 + place is fine when you just say where you live, without adding a duration.

Once you add a duration like 三十天, native speakers strongly prefer 在 + place + 住 + duration: 我在这个城市住了三十天了, not ✗我住在这个城市三十天了.

Is 在 a verb or a preposition in this sentence?

Here 在 behaves like a preposition / coverb meaning “at / in”:

我在这个城市住了三十天了 ≈ “I, in this city, have lived for 30 days now.”

It introduces the location (这个城市) of the action 住, rather than acting as the main verb. The main verb of the sentence is 住 (“to live / reside / stay”).

Why do we say 这个城市 and not just 这城市?

In Mandarin, most nouns need a measure word (classifier) after a demonstrative like 这 (this) or 那 (that).

Standard, neutral: 这个城市 – 这 + 个 + 城市
这城市 is possible in very colloquial or poetic/informal styles, but in normal speech and writing you should use 这个城市.

So here 个 is the default, very common measure word.

Why use 个 as the measure word for 城市? Isn’t 座 also used?

Yes, both are possible, but they feel different:

这个城市 – 个 is the general, neutral classifier; very common in everyday speech.
这座城市 – 座 is a more specific classifier for large, fixed structures (cities, buildings, bridges). It can sound a bit more vivid or literary.

In casual conversation, 这个城市 is completely natural and probably the most common choice.

Why is 天 used instead of 日 for “day” here?

天 is the normal, everyday word for “day” in spoken Chinese:

三十天 – “30 days” (neutral, spoken, standard).

日 is more formal and is common in written language, documents, dates, set phrases, etc.:

三十日 would sound written/official, not how people usually talk about how many days they have lived somewhere.

So 三十天 is the natural choice here.

Why is the duration 三十天 placed after 住了? Could I say 三十天住了 instead?

The typical pattern for duration of an action is:

Verb + 了 + Duration (+ 了)

So here:

住了三十天了 (have lived for 30 days now)

You cannot just put the duration before the verb in this sentence:

✗我在这个城市三十天住了 is ungrammatical.
Even ✗我在这个城市三十天了住 is wrong.

For this meaning, keep: 住了 + 三十天 (+了).

How would I say “I will live in this city for 30 days” (future meaning)?

For a future plan, you usually drop the 了 and add a future marker or context:

我会在这个城市住三十天。 – I will live in this city for 30 days.
我要在这个城市住三十天。 – I’m going to live in this city for 30 days.
In the right context, even 我在这个城市住三十天。 can mean future (“I’ll live here 30 days”), if it’s clearly about a plan.

The double 了 pattern is not used for future; it’s for “have already done … for [duration] (up to now)”.

How would I say “I lived in this city for 30 days (but not anymore)”?

Two common ways:

我在这个城市住了三十天。
– Lived there for 30 days; no sentence‑final 了, so it usually describes a completed period.
我在这个城市住过三十天。
– Uses 过 to stress past experience: “I’ve (once) lived in this city for 30 days.”

Both normally imply that you’re not living there now, unless context says otherwise.

How can I ask “How long have you lived in this city?” using the same pattern?

Use 多长时间 (how long) with the same structure and a final 了:

你在这个城市住了多长时间了？
Literally: “You in this city have-lived how-long time le?”

This asks about a period that started in the past and continues up to now, just like the original sentence but in question form.