Learn Hardcore German: Zu Hause habe ich einen Wasserfilter gekauft, damit ich viel Leitungswasser trinken kann, das gefiltert besser schmeckt. - At home I have bought a water filter so that I can drink a lot of tap water that tastes better filtered.

Zu Hause habe ich einen Wasserfilter gekauft.
→ Focuses on where/for where this is relevant (“At home / For at home, I bought a water filter.”)
Ich habe zu Hause einen Wasserfilter gekauft.
→ More neutral; just tells you that I bought a water filter at home.

In both cases, the verb habe is in second position:

1st version: Zu Hause (1st) – habe (2nd) – ich …
2nd version: Ich (1st) – habe (2nd) – zu Hause …

Both word orders are grammatically correct; they just structure the information differently.

What is the difference between "zu Hause" and "nach Hause"?

They express different ideas:

zu Hause = “at home” (location, state)
- Ich bin zu Hause. – I am at home.
- Zu Hause habe ich einen Wasserfilter. – At home I have a water filter.
nach Hause = “(to) home” (direction, movement toward home)
- Ich gehe nach Hause. – I am going home.
- Ich fahre nach Hause. – I drive home.

So in the sentence, zu Hause makes sense because we’re talking about the situation at home, not about going home.

Why is it "einen Wasserfilter" and not "ein Wasserfilter"?

Because "Wasserfilter" is masculine, and it is the direct object of the verb kaufen, so it must be in the accusative case:

Nominative (subject): der Wasserfilter
Accusative (direct object): den Wasserfilter
With the indefinite article:
- Nominative: ein Wasserfilter
- Accusative: einen Wasserfilter

In the sentence:
Ich habe einen Wasserfilter gekauft.
“I” is the subject, “have bought” is the verb, and einen Wasserfilter is what was bought → accusative.

Why is "Wasserfilter" masculine? How does the compound work?

In German, compound nouns usually take the gender of the last element in the compound.

der Filter (masculine)
das Wasser (neuter)

Wasserfilter = Wasser + Filter → takes the gender of Filter → der Wasserfilter.

This is why in the accusative singular you get einen Wasserfilter (masculine accusative).

Why does the sentence use "habe ... gekauft" instead of just "kaufte"?

German has two common past forms:

Perfekt (present perfect): ich habe gekauft
Präteritum (simple past): ich kaufte

In spoken German, the Perfekt is used for most verbs to talk about completed past actions:

Ich habe einen Wasserfilter gekauft. – I bought a water filter.

The Präteritum (e.g. ich kaufte) is more common in written narratives, or with a small group of very frequent verbs (war, hatte, konnte, musste, etc.) even in speech.

So habe ... gekauft is the natural spoken form here.

Why is it "viel Leitungswasser" and not "viele Leitungswasser"?

Because "Leitungswasser" (tap water) is an uncountable (mass) noun in German.

With uncountable nouns → use viel:
- viel Wasser, viel Kaffee, viel Geld, viel Zeit
With countable plural nouns → use viele:
- viele Gläser Wasser, viele Flaschen, viele Leute

So:

viel Leitungswasser = a lot of tap water (mass)
but:
viele Gläser Leitungswasser = many glasses of tap water (countable).

What exactly does "Leitungswasser" mean, and why is it one word?

Leitungswasser is a compound noun:

die Leitung = pipe, line
das Wasser = water

Leitungswasser literally: “pipe-water” → tap water.

German typically writes such combinations as one word:

Leitungswasser (Leitung + Wasser)
Wasserflasche (Wasser + Flasche)
Haustür (Haus + Tür)

You could also say Wasser aus der Leitung, but Leitungswasser is the standard, compact way to say tap water.

What does "damit" mean here, and how is it different from "um ... zu"?

In this sentence, damit is a subordinating conjunction meaning “so that / in order that” and introduces a purpose clause:

..., damit ich viel Leitungswasser trinken kann, ...
→ “… so that I can drink a lot of tap water …”

You could also express purpose with "um ... zu" when the subject is the same:

..., um viel Leitungswasser trinken zu können.

Differences:

damit + finite clause
- Requires a full clause with subject and verb:
  - damit ich viel Leitungswasser trinken kann
- Slightly more flexible; can also be used when the subject changes.
um ... zu + infinitive
- More compact: um viel Leitungswasser trinken zu können
- Normally used only when the subject is the same as in the main clause.

In this particular sentence, both are grammatically possible. damit makes the purpose very explicitly clausal: “so that I can …”

Why is the word order "trinken kann" and not "kann trinken" in "damit ich viel Leitungswasser trinken kann"?

This is because "damit" introduces a subordinate clause, and in German subordinate clauses have the finite verb at the end.

Main clause:
- Ich kann viel Leitungswasser trinken.
  → finite verb kann in second position.
Subordinate clause (introduced by "damit"):
- ..., damit ich viel Leitungswasser trinken kann.
  → subject ich, objects/adverbs in the middle, finite verb kann at the very end.

When there is a modal verb (kann) and a main verb (trinken), in a subordinate clause the main verb comes directly before the final finite verb:

..., damit ich viel Leitungswasser trinken kann.

So "trinken kann" is the correct order here.

Why is there a comma before "damit" and another before "das"?

In German, commas are mandatory before:

Subordinating conjunctions like damit, weil, dass, wenn, obwohl etc.
- ..., damit ich viel Leitungswasser trinken kann, ...
Relative pronouns (der, die, das, welcher …) when they start a relative clause.
- ..., das gefiltert besser schmeckt.

So both damit and das introduce dependent clauses, and German spelling rules require a comma before those clauses.

Why is the relative pronoun "das" used, and not "es" or "was", in "..., das gefiltert besser schmeckt"?

The relative pronoun must:

Refer back to a specific noun.
Match that noun’s gender and number.

Here, it refers to Leitungswasser, which is neuter singular:

das Leitungswasser

Therefore, the correct relative pronoun is das (neuter singular):

..., das gefiltert besser schmeckt.
→ “… which tastes better when filtered.”

Why not es or was?

es cannot introduce a relative clause; it’s just a normal pronoun (“it”).
was is used after things like alles, nichts, etwas, vieles or after a whole clause, e.g.:
- Alles, was gefiltert ist, schmeckt besser.
- Er hat das gesagt, was mich überrascht hat.

Here we have a concrete noun (das Leitungswasser), so das is the correct relative pronoun.

What exactly is going on in "das gefiltert besser schmeckt"? Why is "gefiltert" in that position?

The relative clause is:

..., das gefiltert besser schmeckt.

Structure (simplified):

das – subject (refers to Leitungswasser)
gefiltert – past participle used as a predicative / descriptive element (“when filtered”)
besser – adverb (“better”)
schmeckt – finite verb (“tastes”)

The idea is:
"das (Leitungswasser), [wenn es] gefiltert [ist], besser schmeckt."
→ “(the tap water) which tastes better when it is filtered.”

So:

gefiltert functions like a descriptive state of the water (“in a filtered state”).
It comes before besser, which is a degree adverb modifying schmeckt.

A more explicit but longer version would be:

..., das besser schmeckt, wenn es gefiltert ist.

But German often uses this kind of compact participle construction:

Gefiltert schmeckt das Wasser besser.
Gekühlt schmeckt der Wein besser.
Getoastet schmeckt das Brot besser.

In the relative clause, the same idea appears as "das gefiltert besser schmeckt".