Ég fylli flösku af vatni áður en ég fer út.

Fylli is the 1st person singular present tense of the verb fylla, meaning to fill.

So:

• ég fylli = I fill
• þú fyllir = you fill
• hann/hún/það fyllir = he/she/it fills

In context, ég fylli can mean either a habitual action (I fill a bottle before I go out) or a present/future-like action (I fill a bottle before I go out / before I’m going out), depending on the situation.

A small extra note: fylli can also be a subjunctive form in other contexts, but here it is understood as a normal present-tense form.

Icelandic does not have an indefinite article like English a/an.

So a bare noun often means a or an:

• flaska = a bottle / bottle
• maður = a man / man

If Icelandic wants to say the bottle, it uses the definite article attached to the noun:

• flaskan = the bottle
• flöskuna = the bottle (accusative singular)

So flösku here naturally means a bottle.

Because flösku is the accusative singular form of flaska.

In this sentence, the bottle is the direct object of the verb fylla:

• Ég fylli hvað? → flösku

So Icelandic uses the accusative.

The basic forms are:

• nominative: flaska
• accusative: flösku

This is very common in Icelandic: nouns change form depending on their grammatical role in the sentence.

This is due to a common Icelandic vowel change often called u-umlaut.

The ending -u affects the stem vowel, so a changes to ö:

• flaska → flösku

You will see this pattern in many words. It is something learners gradually get used to rather than something you can always predict perfectly at first.

So the change is not random: it is part of how the noun is declined.

Because with the verb fylla, Icelandic normally uses the pattern:

• fylla eitthvað af einhverju

That means:

• to fill something with something

So:

• fylla flösku af vatni = fill a bottle with water

This is simply the standard Icelandic construction. Even though English uses with, Icelandic often uses af here.

Because the preposition af takes the dative case in this construction.

The noun vatn is neuter, and its dative singular form is vatni:

• nominative/accusative: vatn
• dative: vatni

So:

• af vatni = with water / literally of water in form, but functionally with water here

A useful rule to remember is that many Icelandic prepositions require a specific case, and af commonly takes the dative.

Áður en means before and introduces a clause.

So:

• áður en ég fer út = before I go out

You can think of it as a conjunction that connects the two actions:

1. filling the bottle
2. going out

It is a very common way to express that one action happens earlier than another.

Because áður en introduces a subordinate clause, and subordinate clauses do not follow the same main-clause word order pattern that often puts the verb in second position.

So after áður en, normal subject-verb order is used:

• áður en ég fer út

not

• áður en fer ég út

This is an important difference between main clauses and subordinate clauses in Icelandic.

Icelandic often uses the present tense for actions that are:

• habitual
• scheduled
• near future
• understood from context

So Ég fylli flösku af vatni áður en ég fer út can mean something like:

• I fill a bottle with water before I go out
• I fill a bottle with water before I leave

English sometimes prefers a future-like idea, but Icelandic does not need a special future tense here. The present tense is enough.

Út means out or outside.

With fara, it forms a very common expression:

• fara út = go out / go outside

So ég fer út means:

• I go out
• I go outside

The word út adds the idea of movement outward, not just the general idea of going.

A word-for-word breakdown is:

• Ég = I
• fylli = fill
• flösku = a bottle (accusative)
• af vatni = with water
• áður en = before
• ég fer út = I go out

So the structure is very close to English overall, except for two especially Icelandic features:

• noun case changes: flaska → flösku
• the construction fylla eitthvað af einhverju for fill something with something