Það er leki undir vaskinum, og vatnið fer núna út á gólfið.

In Icelandic, það er is the normal way to say there is / there are in an existential sense.

So:

• Það er leki = There is a leak

Here það is a dummy subject. It does not literally mean that in this sentence.

Because the sentence is introducing something new and indefinite: a leak, not the leak.

In Icelandic, existential sentences like Það er ... usually take an indefinite noun:

• Það er leki = There is a leak
• Það er maður úti = There is a man outside

If you said lekinn, that would mean the leak, which sounds much less natural here unless the leak had already been identified.

Because undir can take different cases depending on meaning:

• accusative for movement toward/under something
• dative for location under something

Here the leak is located under the sink, so Icelandic uses the dative:

• undir vaskinum = under the sink

The base noun is vaskur = sink.
vaskinum is the dative singular definite form, so it means the sink after undir in this location meaning.

Because Icelandic usually uses a suffixed definite article instead of a separate word.

For example:

• vatn = water

• vatnið = the water

• gólf = floor

• gólfið = the floor

• vaskur = sink
• vaskurinn = the sink

When the noun changes case, the article changes with it. So vaskinum is not just sink plus the; it is a full inflected form meaning the sink in the dative singular.

Because this is not talking about water in general. It means the specific water involved in the leak.

So:

• vatn = water in a general sense
• vatnið = the water, the particular water we are talking about

English often leaves mass nouns bare, but Icelandic commonly uses the definite form when the reference is specific.

This is a very common difference between Icelandic and English.

The verb fara basically means to go, but Icelandic often uses it for things moving from one place to another, including liquids in everyday speech.

So:

• vatnið fer út á gólfið is natural Icelandic

A more physically descriptive verb would be renna:

• vatnið rennur út á gólfið = the water is flowing/running out onto the floor

Both can work, but fer is very normal here.

Because á also changes case depending on whether there is movement or location:

• á + accusative = movement onto/to
• á + dative = location on

Here the water is moving onto the floor, so Icelandic uses the accusative:

• út á gólfið = out onto the floor

If you were describing where the water already is, you would use the dative:

• Vatnið er á gólfinu = The water is on the floor

Út adds the idea of movement out or outward.

So fer út á gólfið suggests that the water is coming out from somewhere and ending up on the floor. In this context, it fits the idea of water leaking out from under the sink.

Without út, fer á gólfið would still mean that it goes onto the floor, but út makes the direction and situation more vivid.

Núna means now / right now. It adds urgency: the water is going onto the floor at this moment.

Its position is fairly natural here:

• vatnið fer núna út á gólfið

But Icelandic adverbs can often move around for emphasis:

• Núna fer vatnið út á gólfið
• Vatnið fer út á gólfið núna

These do not all sound exactly the same in emphasis, but they are all understandable.

Because Icelandic main clauses normally follow the verb-second rule.

That means the finite verb tends to come in the second position of the clause.

In:

• og vatnið fer núna út á gólfið

the first position is vatnið, and the second is the finite verb fer.

If you move another element to the front, the verb still stays second:

• Núna fer vatnið út á gólfið

This is a very important pattern in Icelandic.

These two letters are often difficult for English speakers, but they are actually familiar sounds:

• þ is like th in thin
• ð is like th in this

So:

• það begins with the thin sound
• the final ð in það is the voiced th sound, though in real speech it can be quite light

A few other helpful pronunciation notes from this sentence:

• á is a separate vowel sound, not just plain a
• ú is also its own vowel sound
• the stress in Icelandic usually falls on the first syllable

So even if individual sounds are new, the stress pattern is usually predictable.