Penninn er brotinn.

to be

Breakdown of Penninn er brotinn.

• penninn: the pen
• brotinn: broken

Questions & Answers about Penninn er brotinn.

Why does penn become penninn?

Icelandic often expresses the by attaching a definite article suffix to the noun.

• penni = a pen (indefinite)
• penninn = the pen (definite)
  The base noun here is penni (masculine). In the nominative singular, penni + -inn → penninn.

What case is penninn in, and how can I tell?

Penninn is nominative singular because it is the subject of the sentence (the thing being described). Also, the verb er (from að vera, to be) links a subject to a description, so the subject stays in the nominative.

Why is the verb er used, and what does it mean grammatically?

Er is the 3rd person singular present tense form of að vera (to be). It agrees with the subject (penninn, he/it grammatically).
So: penninn er … = the pen is …

What is brotinn exactly—an adjective or a verb form?