Learn Danish: Det store hus har tredive værelser. - The large house has thirty rooms.

Just like English "thirty" combines "three" and "ty" (tens), tredive is made from tre (3) and an old ending for "tens". Watch out for the pronunciation: the d in tredive is the soft Danish "d", so it sounds a bit like "trah-theh".

Why does the sentence use Det ... hus for "the house" instead of just huset?

Normally, "the house" is huset, attaching the article to the end of the word. But as soon as you put an adjective like store (large) in front of a noun, you must use a separate front word for "the" instead. Since hus is an et-word, we use Det.

Why is there an -e at the end of store (large)?

Adjectives usually get an extra -e at the end when they are used with a definite "the" word like den or det. So the base word stor becomes store.

What is the singular form of værelser (rooms)?

The singular is et værelse (a room). Since the base word already ends in an -e, you only need to add an -r to make it into the plural form værelser.