Für den Fall, dass der Wasserdruck sinkt, wird nachgefüllt.

It means in case or in the event that.

So:

• Für den Fall, dass der Wasserdruck sinkt = In case the water pressure drops

This expression is a bit formal or technical, and it is very common in instructions, manuals, and system descriptions.

A close alternative is:

• Falls der Wasserdruck sinkt, ... = If the water pressure drops, ... / In case the water pressure drops, ...

Because für always takes the accusative case.

• nominative: der Fall
• accusative: den Fall

So:

• für den Fall = for the case / in the event

This is a fixed expression:

• für den Fall, dass ...

Because dass introduces a subordinate clause.

Here, the clause after dass explains which case/event is meant:

• Für den Fall = for the case
• dass der Wasserdruck sinkt = that the water pressure drops

So literally, the structure is something like:

• for the case that the water pressure drops

In natural English, we usually just say in case the water pressure drops.

Because after dass, German uses subordinate-clause word order, where the conjugated verb goes to the end.

So:

• dass der Wasserdruck sinkt

not:

• dass der Wasserdruck sinkt ✅
• dass sinkt der Wasserdruck ❌

This is one of the most important German word-order patterns:

• main clause: verb usually in second position
• subordinate clause (with dass, weil, wenn, etc.): verb usually at the end

Wasserdruck is a compound noun:

• Wasser = water
• Druck = pressure

So Wasserdruck means water pressure.

In German compound nouns, the last part determines the gender. Since der Druck is masculine, the whole word is also masculine:

• der Druck
• der Wasserdruck

That is why the sentence says:

• der Wasserdruck sinkt

sinkt is the present-tense form of sinken, meaning to sink, to drop, or to go down.

Here it means:

• der Wasserdruck sinkt = the water pressure drops

This is a normal verb choice for levels, values, pressure, temperature, and so on.

Examples:

• Die Temperatur sinkt. = The temperature drops.
• Der Pegel sinkt. = The water level falls.

Because this sentence uses the passive voice.

• nachfüllen = to refill / to top up
• wird nachgefüllt = is refilled / refilling is carried out

This is common in technical German when the focus is on what happens, not on who does it.

Compare:

• Man füllt nach. = They/one refill(s).
• Es wird nachgefüllt. = Refilling is done. / It is refilled.

In manuals and system descriptions, German often prefers this kind of passive wording.

Nachfüllen is a separable verb:

• nach-
  • füllen

In a normal main clause, it separates:

• Man füllt nach.

But in forms with a past participle, it comes back together:

• nachgefüllt

So:

• infinitive: nachfüllen
• present main clause: füllt ... nach
• participle: nachgefüllt

That is why the passive is:

• wird nachgefüllt

This is an example of an impersonal passive.

German can use the passive even when no specific object is named, especially in technical or procedural language. The idea is simply that refilling happens.

A fuller version could be:

• Es wird nachgefüllt.

But when another element stands first, this es is usually omitted:

• Für den Fall, dass der Wasserdruck sinkt, wird nachgefüllt.

So the sentence does not need a visible subject in English terms. It is basically saying:

• ... refilling is carried out
• ... more is added
• ... the system refills

depending on context.

Because the first part of the sentence already occupies the first position in the main clause:

• Für den Fall, dass der Wasserdruck sinkt, = position 1

In a German main clause, the conjugated verb must be in second position, so the sentence continues with:

• wird

That gives:

• Für den Fall, dass der Wasserdruck sinkt, wird nachgefüllt.

This is the usual verb-second rule in German.

Compare:

• Es wird nachgefüllt.
• Dann wird nachgefüllt.
• Für den Fall, dass der Wasserdruck sinkt, wird nachgefüllt.

In each case, the finite verb wird is in second position.

Yes, but the tone changes a little.

Possible alternatives:

• Falls der Wasserdruck sinkt, wird nachgefüllt.
• Wenn der Wasserdruck sinkt, wird nachgefüllt.

Differences:

• falls = often very close to in case
• wenn = often just if/when
• für den Fall, dass = more formal and typical of written instructions or technical language

So the original sentence sounds especially suitable for a manual, procedure, or system description.

Because dass der Wasserdruck sinkt is a subordinate clause, and German subordinate clauses are separated by commas.

Also, the introductory phrase plus subordinate clause is followed by the main clause, so a comma is required before the main clause begins:

• Für den Fall, dass der Wasserdruck sinkt, wird nachgefüllt.

German uses commas more consistently than English with subordinate clauses, so this is standard punctuation.

It depends on context.

• refilled is a general translation
• topped up can sound more natural if only a small amount is added
• in technical contexts, replenished may also fit

So wird nachgefüllt could mean:

• it is refilled
• it is topped up
• more is added

The exact best translation depends on what is being filled: a tank, a boiler, a heating system, a reservoir, and so on.