Learn Hardcore Icelandic: Ég fylli glasið með vatni.

glasið is the definite form of glas (“glass”), meaning “the glass.” In Icelandic you attach a suffix to mark definiteness. Here -ið is the neuter singular definite article. If you meant “a glass,” you would simply say glas (accusative indefinite).

Why is vatni in this form?

The base form is vatn (“water”). Because the preposition með (“with”) always takes the dative case, vatn becomes vatni (dative singular). We don’t use vatnið (“the water”) here because the sentence speaks of water in general, not “the water.”

What case is glasið in and how can I tell?

glasið is the direct object of the verb and therefore in the accusative case. For neuter strong nouns like glas, the accusative singular form is spelled exactly like the nominative. The -ið still shows definiteness even though the case ending itself is zero.

Why is the verb fylla changed to fylli here?

fylli is the 1st person singular present indicative form of fylla (“to fill”). In many Icelandic verbs you drop the infinitive ending -a and add -i in the present tense. The full present-tense paradigm of fylla is: • Ég fylli
• Þú fyllir
• Hann/hún fyllir
• Við fyllum
• Þið fyllið
• Þeir/þær/þau fylla

Could you drop the pronoun ég in this sentence?

Yes. Icelandic is a pro-drop language, so it’s perfectly grammatical to say Fylli glasið með vatni. Speakers often include ég for emphasis, clarity or style, but it isn’t strictly required.

Could I use a different preposition instead of með?

A common alternative is af with the dative:
Ég fylli glasið af vatni.
Here af also conveys “fill something with something.” Both með vatni and af vatni are idiomatic, though með tends to sound more straightforward.

What’s the difference between fylla and hella when talking about liquids?

hella means “to pour” (the action of pouring a liquid), whereas fylla means “to fill” (to bring something up to a certain level or capacity). You would say ég helli vatni í glasið (“I pour water into the glass”) to describe the pouring action, and ég fylli glasið með vatni (“I fill the glass with water”) to describe the end state.

What is the word order in Ég fylli glasið með vatni?

The default Icelandic main‐clause order is S–V–O–Adverbial/Prepositional Phrase. So you have:
Subject (Ég) – Verb (fylli) – Direct Object (glasið) – Prepositional Phrase (með vatni).
You can rearrange elements for emphasis (e.g. Með vatni fylli ég glasið).

How do you pronounce með and the ending -ið in glasið?

• með is pronounced [mɛːð] – a long [ɛː] followed by the voiced dental fricative [ð] (like “th” in English “this”).
• In glasið, the -ið is vowel is a short [ɪ], and the final ð is again [ð].