Hún er vön að hella mjólk í kaffið sitt, en í dag vill hún bara te.

It means is used to or is accustomed to doing something.

So Hún er vön að hella mjólk í kaffið sitt means that pouring milk into her coffee is her normal habit. In natural English, this can sound like She usually pours milk into her coffee.

A useful point: Icelandic vera vanur/vön að + infinitive is not limited to past time. With er, it describes a present habit or state.

Because vön is the feminine singular form of the adjective vanur.

The subject is hún, which is feminine singular, so the adjective has to match it:

• vanur = masculine singular
• vön = feminine singular
• vant = neuter singular

So:

• Hann er vanur að...
• Hún er vön að...
• Það er vant að... would be neuter

Because að is the infinitive marker, like English to.

After vera vanur/vön að, Icelandic normally uses að + infinitive:

• vera vön að hella = to be used to pouring
• literally, be accustomed to pour

So að hella is just the normal infinitive construction after vön.

Because mjólk here means milk in a general, uncountable sense.

In Icelandic, as in English, mass nouns often appear without the definite article when you mean the substance generally:

• hella mjólk = pour milk
• drekka vatn = drink water

If you were talking about some specific milk already mentioned, then a definite form could make sense, but here mjólk is the natural choice.

Because that is a normal Icelandic pattern.

English usually prefers her coffee, without the. Icelandic often keeps the definite noun and still adds a possessive:

• kaffið sitt = her own coffee
• bílinn sinn = his/her own car

So this is not strange or redundant in Icelandic. It is just how possession is often expressed.

Because the coffee belongs to the same person as the subject of the sentence.

Icelandic uses the reflexive possessive sinn/sín/sitt when the possessor is the subject:

• Hún drekkur kaffið sitt = She drinks her own coffee

If you said hennar, it would usually mean the coffee belongs to some other woman, not the subject herself:

• Hún drekkur kaffið hennar = She drinks her coffee, where her refers to another female person

So sitt is the correct reflexive form here.

Because sinn/sín/sitt agrees with the thing possessed, not with the owner.

The possessed noun here is kaffið, which is neuter singular, so the possessive must also be neuter singular:

• sinn = masculine singular
• sína / sín forms appear with feminine or plural depending on case
• sitt = neuter singular

So even though the owner is hún, the form is sitt because it matches kaffið.

Because í takes different cases depending on meaning:

• accusative for movement into
• dative for location in

Here there is motion or direction: milk is being poured into the coffee, so accusative is used.

That gives:

• í kaffið = into the coffee
• í kaffinu = in the coffee

So í kaffið is the correct choice in this sentence.

Because Icelandic normally follows a verb-second pattern in main clauses.

That means the finite verb usually comes in the second position. If a time phrase or some other element comes first, the verb still stays second:

• Í dag vill hún bara te
• literally: Today wants she only tea

If the subject comes first, then you get:

• Hún vill bara te í dag

Both are possible, but fronting í dag gives it emphasis: but today...

Because te here means tea in a general, non-specific sense.

She does not want a particular already-known tea; she just wants tea:

• bara te = just tea / only tea
• teið = the tea, a specific tea

So the sentence means that today she simply wants tea, not that she wants some specific tea already identified in the conversation.

Here it describes a present habit or usual behavior, because the verb is er.

So:

• Hún er vön að... = She is used to... / She usually...
• Hún var vön að... = She was used to... / She used to...

This is important because English used to often sounds past-only, but Icelandic er vön að is perfectly normal for a current habit or tendency.