Learn Romanian: Tu dormi douăzeci de ore în acest pat! - You sleep twenty hours in this bed!

This is the main rule for counting in Romanian! For numbers from 1 to 19, the number goes directly before the noun. But starting at the number 20, you must place the word de between the number and the noun. You can think of it as literally saying "twenty of hours".

What is the singular form of ore (hours)?

The singular is oră, which is a feminine noun. In Romanian, many feminine nouns that end in -ă change their ending to -e in the plural, which gives us ore.

Why do we use acest instead of această for "this" before pat?

The word for "this" has to match the gender of the noun it points to. The word pat (bed) is a neuter noun. In the singular form, neuter words use the same matching words as masculine nouns, so we use the masculine/neuter form acest. If the noun were feminine, we would use această.

Since dormi already means "you sleep", do we have to include the word tu?

No, you can drop tu completely. Because the verb dormi specifically means "you sleep", the subject pronoun is optional. Including tu at the start of the sentence just gives it extra emphasis, as if saying, "YOU sleep twenty hours!"