Hún var bara að grínast.

A natural word-for-word breakdown is:

• Hún = she
• var = was
• bara = just
• að grínast = joking / to be joking

So the whole sentence means something like She was just joking.

A very literal version is closer to She was just in the process of joking, but in normal English that becomes She was just joking.

Var is the past tense of vera, which means to be.

So:

• hún er = she is
• hún var = she was

Because the sentence refers to a past situation, Icelandic uses var.

In this sentence, vera að + infinitive is a very common Icelandic pattern used for an ongoing action, similar to the English progressive:

• Hún er að grínast = She is joking
• Hún var að grínast = She was joking

So here að is part of the construction var að grínast, not the preposition to in the English sense.

This is one of the most useful Icelandic patterns to learn:

• ég er að lesa = I am reading
• þau voru að borða = they were eating
• hann er að tala = he is talking

Grínast means to joke, to kid, or to fool around.

In many situations, it is used when someone is not being serious:

• Ég er bara að grínast = I’m just joking
• Ertu að grínast? = Are you joking? / Are you kidding?

So in your sentence, að grínast means that she was saying or doing something playfully, not seriously.

The ending -st often appears in Icelandic verbs that are middle-voice or have a reflexive-like meaning. You do not need to translate it separately every time, but it is an important part of the verb.

So grínast is just the normal dictionary form of this verb. You learn it as a whole item:

• að grínast = to joke

For an English speaker, the safest approach is usually:

• do not try to split it into a separate grína + st
• just memorize grínast as the verb you use for joke

Bara usually means just, only, or simply, depending on context.

In this sentence, it softens the statement and gives the sense:

• She was just joking
• She was only joking

It often makes the sentence sound less serious or more reassuring.

Compare:

• Hún var að grínast = She was joking
• Hún var bara að grínast = She was just joking

Yes, you may hear or use Hún grínaðist bara, and it can also mean She was just joking.

But there is a slight difference in feel:

• Hún var bara að grínast emphasizes the ongoing action or the situation at that moment.
• Hún grínaðist bara is a more simple past statement: She joked / she was joking.

The vera að + infinitive structure is especially common when explaining what was going on in a situation, which is why Hún var bara að grínast sounds very natural.

Normally, Icelandic does not drop subject pronouns the way some languages do.

So you usually say:

• Hún var... = She was...
• Ég er... = I am...
• Þeir komu... = They came...

That means hún is needed here unless the sentence is part of a special structure where the subject is understood from context in some other way.

They are related, but not exactly the same.

• grínast = to joke, to kid around
• gera grín að einhverjum/einhverju = to make fun of someone/something

So:

• Hún var bara að grínast = She was just joking
• Hún var að gera grín að honum = She was making fun of him

This is an important difference, because grínast can be harmless joking, while gera grín að can sound more like teasing or mocking.

A simple pronunciation guide:

• Hún sounds roughly like hoon
• var sounds roughly like var with a short vowel
• bara sounds roughly like BA-ra
• að sounds like a soft ath with voiced th
• grínast sounds roughly like GREE-nast

In Icelandic, the main stress usually falls on the first syllable of a word, so:

• HÚN
• VAR
• BA-ra
• Að
• GRÍ-nast

That gives the sentence a rhythm like:

HÚN var BA-ra að GRÍ-nast

Yes. In natural English, She was just joking and She was just kidding are both good translations.

The best choice depends on tone:

• joking is a bit more neutral
• kidding is often a bit more conversational

So Hún var bara að grínast can match either one very well.