Learn Hardcore German: Seitdem trinke ich nur noch gefiltertes Leitungswasser aus einer großen Flasche. - Since then I only drink filtered tap water from a large bottle.

seit = since (used with a time expression or with a clause):
- seit 2019 – since 2019
- seit ich umgezogen bin – since I moved
seitdem = since then (a temporal adverb, often referring back to a previous sentence or context):
- Ich bin umgezogen. Seitdem trinke ich … – I moved. Since then, I drink…

So in your sentence, seitdem doesn’t take its own verb at the end; it’s not starting a subordinate clause. It’s just an adverb in the main clause, like dann, heute, gestern.

Why is the word order Seitdem trinke ich and not Seitdem ich trinke?

In your sentence, seitdem is being used as an adverb meaning since then, not as a conjunction.

Seitdem trinke ich …
Here seitdem is in the first position (like Heute, Dann). German main clauses must have the finite verb in second position (V2 rule), so the verb trinke comes directly after it, and ich comes third.
Seitdem ich krank war, trinke ich …
Here seitdem is a subordinating conjunction (since), introducing a subordinate clause. In that case, the verb of the subordinate clause goes to the end:
- seitdem ich krank war – since I was ill

So:

Seitdem trinke ich … = main clause, seitdem = adverb
Seitdem ich krank war, trinke ich … = subordinate clause + main clause, seitdem = conjunction

Why is trinke in the present tense? In English I’d probably say “Since then I’ve only drunk …”.

German very often uses the simple present where English would use the present perfect to talk about something that started in the past and continues up to now.

Seitdem trinke ich nur noch …
Literally: Since then I drink only …, but idiomatically in English:
Since then I’ve only drunk / I’ve only been drinking …

The present tense trinke here naturally covers:

habit / repeated action from that time until now
a situation that is still true

So even though English prefers have drunk / have been drinking, German is fine with simple present in combination with seit / seitdem to express that ongoing state.

What exactly does nur noch mean here, and how is it different from just nur or just noch?

nur noch is a very common combination that softens and specifies the meaning:

nur = only
noch = still / no longer anything else / remaining

Together nur noch usually means something like:

only (and nothing else anymore)
now only / still only

In your sentence:

Seitdem trinke ich nur noch gefiltertes Leitungswasser …
→ Since then I (now) drink only filtered tap water, and I don’t drink any other kind of water anymore (e.g. bottled mineral water, unfiltered water).

Compare:

Ich trinke nur gefiltertes Leitungswasser.
I drink only filtered tap water. (No explicit idea that I used to drink something else.)
Ich trinke nur noch gefiltertes Leitungswasser.
I now drink only filtered tap water; I no longer drink anything else.

So noch adds the nuance of a change or a restriction compared to before.

Why is it gefiltertes Leitungswasser and not gefilterte Leitungswasser?

Leitungswasser is a neuter singular noun: das Leitungswasser.

In the sentence:

trinke ich nur noch gefiltertes Leitungswasser

Leitungswasser is the direct object of trinke, so it is in the accusative case, neuter singular.

There is no article in front, so the adjective gefiltert- takes the strong ending for accusative neuter singular: -es.

Declension pattern (strong, no article):

nominative neuter: klares Wasser
accusative neuter: klares Wasser
dative neuter: klarem Wasser

With gefiltert:

(Ich trinke) gefiltertes Leitungswasser. – accusative neuter singular

gefilterte Leitungswasser would suggest plural (“filtered tap waters”), which doesn’t fit, because Leitungswasser is normally uncountable and used in the singular.

Is gefiltertes here an adjective or a participle, and does that change the meaning?

Grammatically, gefiltertes is the past participle of filtern being used adjectivally. This is very common in German:

geöffnetes Fenster – opened/open window
geschriebener Text – written text
gefiltertes Wasser – filtered water

Functionally, you can treat it like an adjective: it describes a property of the Leitungswasser. The fact that it’s originally a participle just tells you that the water has undergone the process of filtering. In practice, gefiltertes Leitungswasser and something like filtriertes Leitungswasser would both be understood as filtered tap water; gefiltert sounds very natural and everyday.

What exactly is Leitungswasser, and how is it different from other words for water?

Leitungswasser literally means pipe water or mains water—in normal English: tap water.

Leitung = pipe, line (here: water pipe)
Wasser = water

So Leitungswasser = water that comes out of the tap at home, in the office, etc.

Contrast with:

Mineralwasser – mineral water (usually bottled)
Tafelwasser – table water (often bottled, sometimes artificially mineralized)
Sprudel(wasser) – sparkling water (colloquial, regional)

So gefiltertes Leitungswasser = filtered tap water.

Why is it aus einer großen Flasche and not some other preposition? What case does aus take?

aus means out of / from the inside of and it always takes the dative case.

In your sentence, the idea is that the speaker drinks the water from a big bottle (the water is inside the bottle, and you pour it out):

aus einer großen Flasche – from a big bottle

Because aus requires dative, Flasche (feminine noun: die Flasche) is in the dative singular feminine:

nominative: eine große Flasche
dative: aus einer großen Flasche

Other prepositions would change the meaning:

von einer großen Flasche – from a big bottle (more like “from the bottle as a source/owner”, not idiomatic for drinking)
in einer großen Flasche – in a big bottle (location, not source)

For drinking, aus einer Flasche trinken is the standard phrase: to drink from a bottle.

Why is it einer großen Flasche and not eine große Flasche?

Because of the dative case.

The preposition aus always takes dative.
Flasche is feminine: die Flasche.
The indefinite article in dative feminine is einer, not eine.

So the forms are:

nominative: eine große Flasche – a big bottle
accusative: eine große Flasche
dative: einer großen Flasche
genitive: einer großen Flasche

Since it’s dative feminine singular, you need:

article: einer
adjective ending: -en → großen

Hence: aus einer großen Flasche.

Can I change the word order, for example: Ich trinke seitdem nur noch gefiltertes Leitungswasser aus einer großen Flasche?

Yes, that version is completely correct and very natural:

Ich trinke seitdem nur noch gefiltertes Leitungswasser aus einer großen Flasche.

German allows you to move adverbials (like seitdem) around, as long as you respect the verb-second rule in main clauses:

Some possible variants:

Seitdem trinke ich nur noch gefiltertes Leitungswasser aus einer großen Flasche.
Ich trinke seitdem nur noch gefiltertes Leitungswasser aus einer großen Flasche.
Nur noch gefiltertes Leitungswasser aus einer großen Flasche trinke ich seitdem. (unusual emphasis, but grammatical)

What you cannot do is put two elements in front of the finite verb so that the verb ends up in third position in a main clause, e.g.
✗ Seitdem ich trinke nur noch … (wrong word order for a main clause).

Could I say nur gefiltertes Leitungswasser noch, or does nur noch have to stay together?

In normal, idiomatic German, nur noch is treated as a fixed combination and generally stays together in front of what it modifies:

Ich trinke nur noch gefiltertes Leitungswasser.

Putting noch at the end, like:

✗ Ich trinke nur gefiltertes Leitungswasser noch.

sounds awkward and is not standard. Native speakers would avoid that.

You can sometimes separate nur and noch in more complex sentences or special emphatic structures, but for learners it’s best to keep nur noch together as a unit meaning only (now) / only anymore.

Could I say Wasser aus der Leitung instead of Leitungswasser?

Yes, that’s possible and understandable:

Wasser aus der Leitung – water from the tap
Leitungswasser – tap water (as a single compound noun)

The compound Leitungswasser is more compact and very common. Wasser aus der Leitung is more descriptive and emphasizes the source. In many contexts they are interchangeable:

Ich trinke nur noch gefiltertes Wasser aus der Leitung.
Ich trinke nur noch gefiltertes Leitungswasser.

Both would be understood as filtered tap water.

Does Seitdem always have to be at the beginning of the sentence?

No. Seitdem is a temporal adverb in this use, so it can be placed in different positions in the main clause, as long as the verb is in second position:

All of these are fine:

Seitdem trinke ich nur noch gefiltertes Leitungswasser aus einer großen Flasche.
(Adverb first for emphasis on the time aspect.)
Ich trinke seitdem nur noch gefiltertes Leitungswasser aus einer großen Flasche.
(More neutral; very common.)
Ich trinke nur noch seitdem gefiltertes Leitungswasser aus einer großen Flasche.
(Possible but sounds a bit odd; the usual place is before nur noch or right after the verb.)

Most natural in everyday speech are (1) and (2).