Learn Hardcore Icelandic: Penninn er þykkur.

Icelandic doesn’t use a separate word for “the” on nouns; instead it attaches a definite‐article suffix. Here penni means “pen,” and adding -inn (masculine, nominative, singular, definite) yields penninn = “the pen.”

How do gender, number, and case affect the form of þykkur?

Adjectives in Icelandic must agree with their nouns in gender, number, and case. Penni is a masculine noun in the nominative singular (and definite), so the adjective þykkur also appears in the masculine nominative singular form to match.

Is þykkur the dictionary form, and is it inflected here?

Yes—þykkur is the lemma (dictionary form) meaning “thick.” In penninn er þykkur, it’s inflected for masculine nominative singular. In this particular case the inflected form looks identical to the lemma.

How would you say “A pen is thick” (indefinite)?

You simply drop the definite suffix on penni:
Penni er þykkur.
Literally “Pen is thick,” i.e. “A pen is thick.”

What is the function of er in this sentence?

er is the present‐tense form of að vera (“to be”), exactly like English “is.” So penninn er þykkur means “the pen is thick.”

How do you pronounce the letter þ in penninn and þykkur?

The letter þ (thorn) is an unvoiced “th” as in English thing ([θ]). The Icelandic y is a rounded front vowel (similar to German ü). So þykkur sounds roughly like “thik‐kur” with [θ], [ɪ], [k], [k], [ʏ], [r].

How would you ask “Is the pen thick?” in Icelandic?

Form a yes/no question by inverting verb and subject:
Er penninn þykkur?

How do you say “the thick pen” if you want the adjective attributively?

Use the demonstrative hinn plus the weak (definite) adjective form:
Hinn þykkri penni = “the thick pen.”