Learn Hardcore German: Das Wasser im Bach ist nicht trinkbar. - The water in the creek is not drinkable.

Also, Wasser is usually an uncountable noun in German, just like water in English. You normally do not say die Wasser to mean “the waters,” except in very poetic or special contexts (e.g. die heiligen Wasser).

What grammatical role does das Wasser have in this sentence?

Das Wasser is the subject of the sentence.

Subject: Das Wasser im Bach
Verb: ist
Predicate adjective: nicht trinkbar

Because it is the subject, das Wasser is in the nominative case. That’s why the article is das, not dem, den, etc.

What does im Bach literally mean, and why is it not in dem Bach?

im is a contraction of in dem. So:

in dem Bach → “in the stream”
im Bach → exactly the same meaning, just shorter and more natural in speech and writing

German very often contracts in dem to im, and an dem to am, especially with places and locations.

Why is it im Bach (dative) and not in den Bach (accusative)?

The preposition in can take dative or accusative, depending on meaning:

Dative → position, location (“where?”)
- im Bach = in dem Bach = in the stream (the water is located there)
Accusative → movement into something (“where to?”)
- in den Bach = into the stream (e.g. Ich springe in den Bach.)

In this sentence, the water is located in the stream, so the dative is correct: im Bach.

What exactly is a Bach, and how is it different from a Fluss?

Both are bodies of flowing water, but:

der Bach = a small natural stream, brook, creek
der Fluss = a larger river

So Bach usually suggests something narrow and not very deep, often in the countryside or mountains. A Fluss is typically wider and more significant (e.g. der Rhein, die Donau).

What are the forms of Bach (gender, cases, plural)?

Bach is masculine: der Bach. Some useful forms:

Nominative: der Bach (Sing.), die Bäche (Pl.)
Accusative: den Bach, die Bäche
Dative: dem Bach, den Bächen
Genitive: des Baches, der Bäche

In the sentence, im Bach = in dem Bach → dem Bach is dative singular.

Where does trinkbar come from, and what does the ending -bar mean?

trinkbar is built from the verb trinken (to drink) + the suffix -bar.

trinken → trink-
- -bar → trinkbar

The suffix -bar usually means “able to be …-ed” or “-able”:

essbar (from essen) = edible
sichtbar (from sehen) = visible
lesbar (from lesen) = readable

So trinkbar means “drinkable / fit for drinking.”

Why is nicht placed before trinkbar and not somewhere else?

In German, when you negate an adjective used as a predicate after sein (to be), nicht normally comes directly before the adjective:

Das Wasser ist trinkbar. → Das Wasser ist nicht trinkbar.
Der Tee ist heiß. → Der Tee ist nicht heiß.

You cannot put nicht at the end the way you sometimes can in English;
✗ Das Wasser im Bach ist trinkbar nicht. is incorrect.

Could I say Das Wasser im Bach kann man nicht trinken instead?

Yes, that is grammatically correct and idiomatic. The meaning is practically the same: the water should not or cannot be drunk.

Nuances:

Das Wasser im Bach ist nicht trinkbar.
- More descriptive, states a property of the water (it is non-drinkable).
Das Wasser im Bach kann man nicht trinken.
- Slightly more action-oriented, focuses on what people cannot/should not do.

In everyday speech, both are fine. The -bar form often sounds a bit more formal or technical.

Is there a word untrinkbar and how is it different from nicht trinkbar?

The word untrinkbar does exist, but it is much less common than nicht trinkbar and can sound stronger or more emotional.

nicht trinkbar = not drinkable / not suitable for drinking
untrinkbar = undrinkable, really impossible or disgusting to drink

You will most often see nicht trinkbar on signs, warnings, etc. (Kein Trinkwasser, Nicht trinkbar).

Why are Wasser and Bach capitalized?

In German, all nouns are capitalized, regardless of their position in the sentence.

das Wasser – noun → capitalized
der Bach – noun → capitalized

Adjectives like trinkbar and particles like nicht are not capitalized unless they start the sentence.

How do you pronounce Bach, especially the ch?

In Bach, the ch is the so‑called ach-sound (like in Dach, Nacht). It does not sound like English k or sh.

Approximate description:

Start with a as in “father.”
Then make a voiceless, raspy sound in the back of your throat, similar to the ch in Scottish loch or German composer Bach.

So Bach is pronounced roughly like bahkh, with that guttural ch at the end.