Jeg fylder glasset med vand, før jeg går i seng.

Because glasset means the glass, while et glas means a glass.

In Danish, the definite article is usually added to the end of the noun:

• et glas = a glass
• glasset = the glass

So Jeg fylder glasset med vand means you are talking about a specific glass, not just any glass.

Since glas is a neuter noun (et-word), the definite ending is -et:

• glas → glasset

Because fylder is the present tense form of the verb at fylde.

A very common Danish pattern is:

• infinitive: at fylde = to fill
• present tense: fylder = fill / am filling / fills

So:

• Jeg fylder = I fill / I am filling

Danish present tense often corresponds to both the English simple present and present continuous, depending on context.

Danish normally uses med after fylde, just like English uses with in fill something with something.

Pattern:

• fylde noget med noget

Examples:

• Jeg fylder glasset med vand.
• Hun fylder flasken med mælk.

So med vand simply tells you what the glass is being filled with.

Because the sentence has two clauses, and each clause needs its own subject.

• Main clause: Jeg fylder glasset med vand
• Subordinate clause: før jeg går i seng

In English, you also repeat the subject:

• I fill the glass with water before I go to bed.

So the second jeg is necessary because jeg går i seng is a full clause of its own.

Because før introduces a subordinate clause, and Danish subordinate clauses usually keep the basic order subject + verb.

So you get:

• før jeg går i seng

not:

• før går jeg i seng

Compare:

• Main clause: Jeg går i seng.
• Subordinate clause: ...før jeg går i seng.

This is an important contrast in Danish:

• In main clauses, word order can change because of the V2 rule
• In subordinate clauses, the order is usually more straightforward: subject before verb

Gå i seng is an idiomatic expression meaning to go to bed.

Literally, it looks more like go in(to) bed, but the natural English meaning is go to bed.

Common forms:

• jeg går i seng = I go to bed
• hun gik i seng = she went to bed
• vi skal gå i seng = we are going to bed / we have to go to bed

So it is best learned as a fixed expression: at gå i seng.

Yes, in many situations you can say inden jeg går i seng.

Both can mean before I go to bed, but there can be a slight nuance:

• før = before
• inden = before / by the time / prior to

In many everyday sentences like this one, they are both natural:

• Jeg fylder glasset med vand, før jeg går i seng.
• Jeg fylder glasset med vand, inden jeg går i seng.

If you are a learner, før is a very safe and common choice.

It can mean either, depending on context, because Danish present tense is flexible.

Jeg fylder glasset med vand can mean:

• I fill the glass with water = habitual
• I am filling the glass with water = happening now

With før jeg går i seng, the sentence often sounds like a habit or a usual routine:

• something you do before going to bed

But context always decides the exact meaning.

Because Danish often places a comma before a subordinate clause.

Here, før jeg går i seng is a subordinate clause introduced by før, so the comma is natural:

• Jeg fylder glasset med vand, før jeg går i seng.

You may notice some variation in comma style when reading Danish, but using the comma here is standard and helpful for learners.

The basic forms are:

• jeg = I
• at fylde = to fill
• et glas = a glass
• vand = water
• før = before
• at gå i seng = to go to bed

And the forms used in the sentence are:

• fylder = present tense of at fylde
• glasset = definite form of et glas
• går = present tense of at gå

This is useful because Danish often changes words by adding endings rather than using separate articles like English does.

Because vand is being used as an uncountable substance noun, like water in English.

You normally would not say:

• med et vand

Just as in English, you simply say:

• with water

So:

• med vand = with water

If you wanted to talk about a specific water in a special context, Danish could use a definite form, but in an ordinary sentence like this, plain vand is exactly what you want.