Learn Icelandic: Er skyrtan of víð? - Is the shirt too loose?

No. of is an adverb of degree meaning "too" or "excessively". Because it is an adverb describing the adjective víð, it never changes its form. It stays exactly the same whether you are talking about a masculine, feminine, or neuter word.

Is the Icelandic word of related to the English word "of"?

No, it is a false friend! In Icelandic, of means "too" (as in "too loose"). If you want to translate the English word "of", you will usually use the Icelandic word af.

Why is the adjective for "loose" written as víð here?

Adjectives must match the gender of the noun they describe. The base noun here is skyrta (shirt), which is a feminine word. Therefore, the adjective takes the feminine singular form, víð. If the noun were masculine, you would use víður, and if neuter, vítt.

How do we know skyrtan means "the shirt" and not just "a shirt"?

In Icelandic, the word for "the" (the definite article) is attached to the end of the noun rather than being a separate word in front of it. The base word for "a shirt" is skyrta. By adding -n to the end, it becomes skyrtan ("the shirt").