Questions & Answers about Stelpan er fljótust, en hún er líka þreyttust.
Why do fljótust and þreyttust end in -ust instead of the regular superlative -ast?
The base superlative ending is indeed -ast-. However, because they are describing a feminine singular subject (stelpan and hún), a vowel change rule called the u-shift turns that a into a u. Therefore, the feminine singular strong form for regular superlatives always ends in -ust.
Since stelpan means 'the girl' (definite), shouldn't the adjective be in the weak declension like fljótasta?
When an adjective comes after the verb to be (like er here) to describe the subject, it usually takes the strong declension, even if the noun it describes is definite. That is why we use the strong forms fljótust and þreyttust instead of the weak forms.
What are the original base forms of these adjectives?
The base (dictionary) forms are fljótur (fast) and þreyttur (tired). To make them superlative, you drop the masculine -ur ending to get the stems fljót- and þreytt-, and then add the superlative endings.
Why is the word for 'the girl' stelpan? Doesn't the feminine definite article usually end in -in?