Wasser dringt in den Keller.

Because in is a two-way preposition in German. It can take:

• accusative when there is movement toward a destination
• dative when there is location and no movement

Here, the water is moving into the basement, so German uses the accusative:

• Wasser dringt in den Keller. = water is getting into the basement

Compare:

• Das Wasser ist im Keller. = the water is in the basement

So:

• in den Keller = into the basement
• im Keller = in the basement

dringt is the 3rd person singular present tense of dringen.

Basic conjugation in the present:

• ich dringe
• du dringst
• er/sie/es dringt
• wir dringen
• ihr dringt
• sie/Sie dringen

In this sentence, the subject is Wasser, which is grammatically singular, so the verb must be dringt.

Here, dringen means something like:

• to penetrate
• to get into
• to force its way into
• to seep into or push into, depending on context

It is often used for things like:

• water
• smoke
• moisture
• light
• sound
• gas

So Wasser dringt in den Keller suggests that water is getting into the basement in an unwanted or forceful way.

German often uses mass nouns like Wasser, Milch, Brot, Geld, etc. without an article when speaking about the substance in a general or indefinite way.

So:

• Wasser dringt in den Keller. = Water is getting into the basement.

This sounds natural because it means some water / water in general, not a specific previously identified water.

You could use an article in a different context, for example:

• Das Wasser dringt in den Keller. = The water is getting into the basement.

That would usually refer to specific water already known from the context.

It is singular.

Even though English sometimes treats words like water as an uncountable substance, German still treats Wasser as a singular noun grammatically. That is why the verb is singular:

• Wasser dringt ...
• not Wasser dringen ...

German does have a plural form Wässer, but that is uncommon and usually means different kinds of waters in special contexts, not ordinary water in a sentence like this.

Because this is a main clause, and in German main clauses the finite verb normally goes in second position.

So the structure is:

• Wasser = first element
• dringt = finite verb in second position
• in den Keller = the rest of the sentence

This is one of the most important German word-order rules.

For example, you could also put another element first:

• In den Keller dringt Wasser.

Now in den Keller is in first position, and the verb dringt is still in second position.

Yes. A very common alternative is:

• Wasser dringt in den Keller ein.

This uses the separable verb eindringen = to penetrate / to get into / to intrude into.

Both versions are possible:

• Wasser dringt in den Keller.
• Wasser dringt in den Keller ein.

The version with ein often makes the idea of entering/intruding a little more explicit.

Notice the word order with the separable verb in a main clause:

• dringt ... ein

So the prefix ein goes to the end.

dringt is in the present tense.

In German, the present tense can cover several English ideas, depending on context, such as:

• water is getting into the basement
• water gets into the basement
• water is penetrating into the basement

In a real-life situation, this sentence would most often mean something happening right now or currently.

Because in German, all nouns are capitalized.

So:

• Wasser is a noun
• Keller is a noun

This is a standard rule of German spelling, not something special about this sentence.

Keller is masculine:

• der Keller = nominative
• den Keller = accusative
• dem Keller = dative

In this sentence, you see den Keller because:

1. Keller is masculine
2. in takes the accusative here because it expresses movement into the basement

So the form den Keller tells you both the gender and the case being used.