Questions & Answers about Sú er með fallegt bros.
Why does the sentence start with Sú instead of Hún (she)?
Sú is the feminine nominative form of the demonstrative pronoun sá (that/the one). While hún just means 'she', using sú points the person out more emphatically, like saying 'Now that one has a beautiful smile' or 'That woman over there'.
Could I use Sá at the beginning of this sentence instead?
Yes, but it would change who you are pointing at. Sú is feminine, so you are pointing out a female. If you were talking about a man or boy, you would use the masculine form Sá, making the sentence Sá er með fallegt bros.
Why does this sentence use er með (is with) instead of the verb hefur (has)?
When talking about physical features, items you are currently carrying, or even illnesses, Icelandic usually uses vera með (to be with) instead of hafa (to have). Because a smile is a physical feature on someone's face, er með is the most natural choice here.
What case is the phrase fallegt bros in?
It is in the accusative case. The preposition með forces the noun that follows it into the accusative case when it means 'having' or 'carrying'. Because bros (smile) is a neuter noun, its accusative form looks exactly the same as its dictionary form, and the adjective fallegt matches it.