Brosið hennar er fallegt.

Bros is the basic noun, meaning smile.
Brosið means the smile.

Icelandic usually adds the definite article to the end of the noun, instead of putting a separate word like the in front.

So:

• bros = smile
• brosið = the smile

In this sentence, brosið hennar is literally the smile of her, which is how Icelandic commonly expresses her smile.

Hennar is the genitive form connected with hún meaning she.

Here it shows possession, so it means her in the sense of belonging to her.

So:

• hún = she
• hennar = her / hers / of her, depending on context

In brosið hennar, it means her smile.

In Icelandic, possessive words like hennar, hans, and þeirra often come after the noun.

So Icelandic commonly says:

• brosið hennar = her smile
• literally: the smile her

This is normal Icelandic word order. For an English speaker, it can feel backwards at first, but it is a very common pattern.

English usually says her smile without the. Icelandic often keeps the noun definite anyway, even when a possessive follows.

So brosið hennar is completely normal Icelandic.

Literally, it looks like:

• the smile of her

But naturally, it means:

• her smile

This is one of those places where Icelandic structure does not match English structure directly.

Fallegt is the form of fallegur meaning beautiful that agrees with brosið.

In Icelandic, adjectives must agree with the noun in:

• gender
• number
• case

The noun bros is a neuter noun, and here it is singular. So the adjective must also be neuter singular:

• masculine: fallegur
• feminine: falleg
• neuter: fallegt

That is why the sentence says Brosið hennar er fallegt, not fallegur or falleg.

Er is the present tense of vera, the verb to be.

So:

• vera = to be
• er = is

In this sentence:

• Brosið hennar er fallegt = Her smile is beautiful

It works just like English is.

The subject is brosið hennar.

That whole phrase means her smile.
The verb is er, and fallegt describes the subject.

So the sentence breaks down like this:

• Brosið hennar = subject
• er = verb
• fallegt = predicate adjective

In other words:

• Her smile = subject
• is = verb
• beautiful = description

Hennar is in the genitive case.

In Icelandic, possession is very often expressed with the genitive. So when you want to say something like her smile, his car, or their house, Icelandic often uses a genitive form like this.

Here:

• hennar = genitive, showing ownership or association

So brosið hennar literally has the sense the smile of her.

In ordinary modern Icelandic, brosið hennar is the natural way to say her smile.

A form like hennar bros is not the usual neutral pattern in everyday language. It may sound marked, literary, or unnatural depending on context.

So for a learner, the safe and standard choice is:

• brosið hennar

A very literal breakdown is:

• Brosið = the smile
• hennar = of her / her
• er = is
• fallegt = beautiful

So the sentence is literally:

• The smile of her is beautiful

But the natural English translation is:

• Her smile is beautiful

It can cover a range of positive meanings like beautiful, pretty, lovely, or nice-looking, depending on context.

In this sentence, beautiful is a very natural translation. With something like a smile, English might also sometimes say:

• Her smile is lovely
• She has a beautiful smile

But grammatically, the Icelandic sentence is simply describing the smile as fallegt.

Yes. It is a very common and basic Icelandic pattern:

• [subject] + er + [adjective]

For example, Icelandic often builds sentences like this:

• Húsið er stórt = The house is big
• Barnið er glatt = The child is happy
• Brosið hennar er fallegt = Her smile is beautiful

So this sentence is a very useful model for learning how Icelandic links a subject to an adjective with er.