Learn Hardcore German: Dank des Wasserfilters steht immer eine Flasche mit gefiltertem Wasser auf meinem Schreibtisch, wenn ich für die Klausur lerne. - Thanks to the water filter there is always a bottle with filtered water on my desk when I study for the exam.

You may see dank with the dative in spoken or less formal German (dank dem Wasserfilter), but the genitive is more standard and is what is used in your sentence.

What is the subject of the sentence and why is the verb steht?

The subject is eine Flasche mit gefiltertem Wasser (“a bottle of filtered water”).

steht is 3rd person singular of stehen (“to stand”).
In German, for objects placed somewhere upright, you often use stehen instead of sein (“to be”).

So instead of saying “A bottle is on my desk”, German naturally says Eine Flasche steht auf meinem Schreibtisch (“A bottle stands on my desk”). That is why the verb is steht, not ist.

Why is the word order Dank des Wasserfilters steht immer eine Flasche ... and not ... eine Flasche steht immer ...?

German main clauses follow the V2 rule: the finite verb must be in the second position (counting chunks, not individual words).

Here, the first position is the prepositional phrase Dank des Wasserfilters.
Because that is in first position, the verb steht must come next in second position.
Then comes the rest of the information: immer eine Flasche mit gefiltertem Wasser auf meinem Schreibtisch.

You could also say Eine Flasche mit gefiltertem Wasser steht dank des Wasserfilters immer auf meinem Schreibtisch. The meaning is the same; only the emphasis changes.

Why is it eine Flasche (nominative) but mit gefiltertem Wasser (dative)?

Eine Flasche is the subject of the sentence, so it must be in the nominative case:

eine Flasche = nominative feminine singular.

The phrase mit gefiltertem Wasser is governed by the preposition mit, and mit always takes the dative case.
So:

das Wasser (nominative / accusative singular)
dem Wasser (dative singular)

And the whole phrase mit gefiltertem Wasser means “with filtered water” in the dative case.

Why does gefiltertem end in -em in mit gefiltertem Wasser?

Gefiltert is an adjective here (participle used like an adjective) and must agree with the noun Wasser in gender, number, and case.

Wasser is neuter, singular.
The preposition mit requires dative.
Dative neuter singular after an article or in such a phrase takes the ending -em.

So the pattern is:

(mit) dem gefilterten Wasser → article present: adjective ending -en
(mit) gefiltertem Wasser → no article: strong ending -em

In your sentence there is no article before Wasser, so you get gefiltertem (strong dative neuter: -em).

Why is there no article before Wasser in mit gefiltertem Wasser?

Wasser is a mass noun (like “water” in English), and in German it is very common to use mass nouns without an article when you talk about them in a general or indefinite way:

Ich trinke Wasser. – “I drink water.”
mit gefiltertem Wasser – “with filtered water.”

An article would be used if you want to be more specific, e.g. mit dem gefilterten Wasser (“with the filtered water” – a particular water already known from context). In your sentence, the general idea “filtered water” is meant, so no article is needed.

Why is it auf meinem Schreibtisch and not auf meinen Schreibtisch?

Auf is a two-way preposition; it can take dative or accusative:

dative = location (where something is)
accusative = direction (where something is going / being put)

Here the bottle is located on the desk (no movement to the desk), so you need the dative:

der Schreibtisch → dative: dem Schreibtisch
with mein-: meinem Schreibtisch

If you described movement to the desk, you would use the accusative:

Ich stelle die Flasche auf meinen Schreibtisch. – “I put the bottle onto my desk.”

Why is immer placed before eine Flasche and not after the verb, like steht eine Flasche immer ...?

Both are grammatically correct:

Dank des Wasserfilters steht immer eine Flasche ...
Dank des Wasserfilters steht eine Flasche immer ...

The difference is subtle and about emphasis and rhythm.

Putting immer early (right after the verb) often sounds more natural in spoken German and emphasizes the frequency:

steht immer eine Flasche → “there is always a bottle”

Placing immer later can emphasize “a bottle” a bit more, but in many contexts, both versions are almost interchangeable.

Why does the verb move to the end in wenn ich für die Klausur lerne?

Wenn introduces a subordinate clause. In German, in subordinate clauses the finite verb goes to the very end of the clause.

So the structure is:

wenn (subordinating conjunction)
ich (subject)
für die Klausur (prepositional phrase)
lerne (verb at the end)

If you made this clause a main clause, it would be Ich lerne für die Klausur. (V2 word order). With wenn, it becomes wenn ich für die Klausur lerne (verb-final).

What is the difference between wenn and wann? Could I say wann ich für die Klausur lerne?

You cannot replace wenn with wann in this sentence.

wenn is used for “when / whenever” in the sense of a condition or a repeated situation:
- Wenn ich für die Klausur lerne, ... – “When(ever) I study for the exam, ...”
wann is used for questions about time:
- Wann lernst du für die Klausur? – “When do you study for the exam?”
- Ich weiß nicht, wann ich für die Klausur lerne. – “I don’t know when I’ll study for the exam.”

In your sentence, you have a conditional/temporal clause “whenever I study for the exam”, so wenn is correct.

What is Klausur exactly? Is it the same as Prüfung?

Both refer to some kind of exam, but they are not identical:

Klausur usually means a written exam, often at school or university, held at a fixed time, often in class or in an exam hall. It is typically time‑limited and supervised.
Prüfung is a more general word for exam / test, and can be written, oral, practical, etc.

So für die Klausur lernen suggests specifically preparing for a particular written exam (e.g. a midterm, final, or test).

Why is it lerne and not studiere in wenn ich für die Klausur lerne?

In German:

lernen means to learn / to study in the sense of preparing, doing homework, revising material.
studieren is more about being enrolled at a university or studying a subject in general, e.g. Ich studiere Physik – “I study physics” (as my degree).

When you talk about preparing for an exam, you normally use lernen:

für die Klausur lernen – to study for the exam.

So wenn ich für die Klausur lerne is the natural phrasing here; studiere would sound wrong in this context.

Can the wenn‑clause also come at the beginning of the sentence, and what happens to the word order then?

Yes, you can put the wenn‑clause first. The whole wenn‑clause then counts as the first position, and the main clause must still obey the V2 rule, so the verb comes directly after the comma:

Wenn ich für die Klausur lerne, steht dank des Wasserfilters immer eine Flasche mit gefiltertem Wasser auf meinem Schreibtisch.

Notice that steht comes right after the comma, because it must be in second position of the main clause, even though the subordinate clause comes before it.