Learn Icelandic: Ég er mjög hrifinn af þessum leikritum.

The preposition af always forces the following words to take the dative case. Since we are talking about "these plays" (plural), we need the dative plural form of the demonstrative pronoun þessi, which is þessum.

Does the word þessum change depending on the gender of the noun?

No! One of the nice things about the dative plural in Icelandic is that it looks the same for all three genders. Whether the following noun is masculine, feminine, or neuter, "these" in the dative plural is always þessum.

Why does the word leikritum end in -um?

Because it follows af, the noun leikrit (play) must also be in the dative plural to match þessum. In Icelandic, almost all nouns in the dative plural end in -um, which makes them easy to spot!

Does hrifinn af always mean "impressed by"?

It can mean "impressed by," but it is also very commonly used to mean "fond of" or "keen on" something. As a bonus tip: if you say ég er hrifinn af þér to a person, it usually means "I like you" or "I have a crush on you!"

If a woman is saying this sentence, does she need to change any words?