Ég er að fylla glasið með vatni.

I am filling the glass with water.

Breakdown of Ég er að fylla glasið með vatni.

• ég: I
• með: with
• glasið: the glass
• vatnið: the water
• fylla: to fill
• vera að: to be (doing)

Questions & Answers about Ég er að fylla glasið með vatni.

Why does the sentence use Ég er að fylla... instead of just the present tense Ég fylli...?

Ég er að + infinitive is a very common Icelandic way to express an action in progress (similar to English I am filling...).
Ég fylli glasið can mean I fill the glass (habitually / generally) or sometimes I am filling the glass depending on context, but Ég er að fylla... makes the “right now / in progress” meaning explicit.

What exactly is the function of að here?

In vera að + infinitive, að introduces the infinitive and is part of this progressive-like construction. So að fylla is essentially “to be in the process of filling.”
You’ll see að with many infinitives, but the specific pattern er að + infinitive is especially frequent in everyday speech.

Why is it fylla and not fylli?

fylla is the infinitive (to fill). In the construction Ég er að + infinitive, the main verb stays in the infinitive form: er að fylla.
fylli is a present-tense form (I fill / I am filling depending on context), used when you’re not using the er að construction.

What case is glasið, and why?

glasið is the direct object of fylla (“fill what?”), and fylla takes a direct object in the accusative.
So it’s ég fylli glasið (accusative singular, definite).

What does the -ið ending in glasið mean?

-ið is the definite article attached to the noun (common in Scandinavian languages).

• glas = a glass
• glasið = the glass

So this sentence specifically refers to a particular glass (“the glass”), not just any glass.

Why is it með vatni and not með vatn?

The preposition með governs the dative case in Icelandic.
vatn (water) in the dative singular is vatni, so með vatni = “with water.”

Is vatni singular or plural here? And what’s the base form?

vatni is dative singular of the noun vatn (“water”).
The base (dictionary) form is vatn (nominative/accusative singular). The dative adds -i: vatn → vatni.

Why does Icelandic say “fill the glass with water” instead of “fill the glass (up)” or “fill the glass of water”?

Icelandic commonly expresses the “contents” using fylla X með Y = “fill X with Y.”
Other natural variations exist, for example:

• Ég er að fylla glasið. = “I’m filling the glass (up).” (contents not stated)
• Ég er að fylla glas af vatni. can occur, but fylla glasið með vatni is a very standard, straightforward way to say it.

Could the word order be different?

Yes, some rearrangements are possible, but the given order is the neutral, most common one:

• Ég er að fylla glasið með vatni. (neutral) You might move parts for emphasis, but Icelandic word order is more constrained than English in many cases, and learners are usually safest sticking to the neutral pattern.

How is Ég er að fylla glasið með vatni pronounced (especially ð and ll)?

A rough guide:

• Ég sounds like yeh(g) (the g is very soft).
• er is like ehr.
• að: the ð is like the th in this (a voiced “th”), and the vowel is short.
• fylla: the y is like a front rounded vowel (similar to German ü). The ll is a special Icelandic sound (often a kind of tl-like or “voiceless l” quality depending on speaker and context), not the English l.
• glasið: final ð again like th in this (often very light at the end).
• vatni: a is short; final i is like a short i.

(If you want, I can give IPA for each word too.)