Questions & Answers about Hitinn er tuttugu og ein gráða í dag.
Does tuttugu change its ending because gráða is a feminine word?
No, tuttugu (twenty) is a number that never changes for gender or case. It is completely indeclinable. In a compound number like 21, only the "one" part changes to match the gender of the noun.
Why is gráða (degree) singular when the temperature is 21? Shouldn't it be plural?
This is a very important rule in Icelandic! Whenever a number ends in 1 (like 21, 31, 41), the noun that follows it is always singular. You are literally saying "twenty and one degree". That is why we use gráða (singular) and not gráður (plural).
Why do we use ein here instead of the default einn?
What is the base form of Hitinn?
The base noun is hiti, which means "heat" or "temperature". It is a masculine word. The -nn at the end is the attached definite article ("the"), so hitinn simply means "the temperature".