Learn Icelandic: Hvaða orð er þetta?

Hvaða is used when you are asking "what" or "which" directly before a noun. Since we are asking "what word" (hvaða orð), we must use hvaða. If you just wanted to ask "What is this?" without naming the object, you would use hvað (Hvað er þetta?).

Do I need to change the ending of hvaða to match the gender of the noun?

No, you don't! Hvaða is a fantastic word for beginners because it never changes form. It stays exactly the same no matter the gender or number of the noun that follows it.

How do we know orð means "word" and not "words" in this sentence?

Orð is a neuter noun that happens to look exactly the same in both the singular and the plural. You can tell it means the singular "word" here because of the verb er (is) and the singular pronoun þetta (this). If it were plural, the sentence would use eru (are).

Why does the verb er come before þetta?

In Icelandic questions, the verb usually takes the second position. The entire question phrase Hvaða orð (What word) acts as the first block of information, so the verb er (is) has to come immediately after it, followed by þetta (this).