Køkkenrullen er lige så praktisk som vaskekluden, når der ligger vand på gulvet.

The -en is the definite article in Danish, attached to the end of many common-gender nouns.

• en køkkenrulle = a kitchen roll / paper towel roll
• køkkenrullen = the kitchen roll
• en vaskeklud = a washcloth / cleaning cloth
• vaskekluden = the washcloth

So instead of putting a separate word like English the before the noun, Danish often adds it to the noun itself.

Lige så ... som means just as ... as or simply as ... as.

In this sentence:

• lige så praktisk som = just as practical as

This is the standard Danish pattern for equality in comparisons:

• Han er lige så høj som sin bror. = He is as tall as his brother.
• Denne bog er lige så god som den anden. = This book is as good as the other one.

So the structure is:

• lige så + adjective + som

Here praktisk is the normal dictionary form of the adjective, and it is used because it comes after er in a comparison.

In Danish, adjectives are often not inflected after verbs like være (to be) in this kind of structure:

• Bilen er hurtig. = The car is fast.
• Huset er stort. = The house is big.
• Køkkenrullen er lige så praktisk som vaskekluden.

You may know adjective endings like -t and -e, but those are used in other contexts, especially when the adjective directly modifies a noun:

• en praktisk løsning = a practical solution
• et praktisk redskab = a practical tool
• praktiske ting = practical things

Here når means when in a general, repeated, or typical situation.

• når der ligger vand på gulvet = when there is water on the floor

Danish often uses når for things that happen regularly or whenever a situation occurs.

Compare:

• Når det regner, bliver vejen glat. = When it rains, the road gets slippery.
• Hvis det regner i morgen, bliver vi hjemme. = If it rains tomorrow, we’ll stay home.

So:

• når = when / whenever
• hvis = if

In your sentence, it means something like whenever there is water on the floor.

This is a very common Danish structure. Der is often used as a kind of introductory or dummy subject, similar to English there in there is / there are.

So:

• der ligger vand på gulvet literally = there lies water on the floor
• natural English = there is water on the floor

This pattern is common when introducing the existence or presence of something:

• Der står en kop på bordet. = There is a cup on the table.
• Der sidder en mand i bilen. = There is a man sitting in the car.
• Der ligger vand på gulvet. = There is water on the floor.

Danish often uses different position verbs where English would simply use is.

Here, ligger literally means lies / is lying, but in many situations it is just the natural verb for something being located somewhere, especially if it is spread out or resting on a surface.

So:

• der ligger vand på gulvet literally = there lies water on the floor
• idiomatic English = there is water on the floor

Other common Danish position verbs are:

• stå = stand
• sidde = sit
• ligge = lie

Examples:

• Bogen ligger på bordet. = The book is on the table.
• Koppen står på bordet. = The cup is on the table.
• Han sidder i stolen. = He is sitting in the chair.

English speakers often need time to get used to this, because Danish is more specific about position.

På gulvet means on the floor.

• på = on
• gulvet = the floor

So:

• på gulvet = on the floor

Danish uses på in many of the same ways English uses on:

• på bordet = on the table
• på væggen = on the wall
• på gulvet = on the floor

Yes. Køkkenrulle is a compound noun, which is extremely common in Danish.

It is made from:

• køkken = kitchen
• rulle = roll

Together:

• køkkenrulle = kitchen roll / paper towel roll

And then with the definite ending:

• køkkenrullen = the kitchen roll

Danish forms many everyday words this way, just like German and often like English compounds:

• vaskeklud = washing cloth / washcloth
  • vaske = wash
  • klud = cloth

Learning to spot compounds makes Danish much easier.

It can mean slightly different things depending on context.

Vaskeklud can refer to:

• a washcloth
• a cleaning cloth
• sometimes a dishcloth-like cloth, depending on the situation

In this sentence, since there is water on the floor, it is probably being used in the sense of a cloth for wiping up water, so cleaning cloth or washcloth both make sense depending on the exact context.

Because after når, Danish normally keeps the usual subordinate-clause word order:

• når der ligger vand på gulvet

Here:

• når = conjunction
• der = subject-like element
• ligger = verb

In subordinate clauses, Danish does not use the main-clause inversion pattern you often see after an adverb at the beginning of a sentence.

Compare:

• Der ligger vand på gulvet. = main clause
• ... når der ligger vand på gulvet. = subordinate clause

If it were a main clause with something placed first, then inversion would happen:

• På gulvet ligger der vand.

But after når, the clause is subordinate, so der ligger is the normal order.

Not really if you mean a kitchen roll / paper towel roll.

• køkkenrulle is the usual word for a kitchen paper roll
• papirrulle would more literally mean paper roll, which is broader and less natural in this context

So if you mean the household item used to absorb spills, køkkenrulle is the best choice.

A fairly literal version would be:

The kitchen roll is just as practical as the washcloth, when there lies water on the floor.

But that is not natural English. A more natural English version would be:

The kitchen roll is just as practical as the washcloth when there is water on the floor.

This is a good example of how Danish and English may be close in structure, but Danish often uses ligge where English prefers be.

A rough pronunciation guide is:

• køkkenrullen ≈ KYUK-en-rool-en
  but that is only approximate

A few points:

• ø is a rounded vowel that does not exist in standard English
• kk makes the consonant sound short and crisp
• rulle has a short u
• the final -en is usually light and unstressed

If you want to imitate it more closely:

• køkken sounds roughly like KYUHK-en
• rullen sounds roughly like ROOL-en with a Danish r

The biggest challenge for English speakers is usually ø. It is somewhat like saying e in her while rounding your lips.

This sentence is useful because it combines several very common Danish patterns:

1. Definite nouns with endings

   • køkkenrullen, vaskekluden, gulvet

2. Equality comparison

   • lige så + adjective + som

3. Subordinate clause with når

   • når der ligger vand på gulvet

4. Existence/location with der

   • der ligger ...

5. Position verb instead of plain is

   • ligger for something located on a surface

So if you understand this one sentence well, you are learning several very common features of everyday Danish at once.