Learn Icelandic: Læknirinn lagaði handlegginn í gær. - The doctor fixed the arm yesterday.

Handleggur (arm) is a masculine noun. In the accusative case (used here because it is the object being fixed), the -ur ending drops, leaving handlegg. When you add the masculine accusative definite article (the suffix for 'the'), you add -inn, making it handlegginn.

Both læknirinn and handlegginn end in -inn. Are they in the same case?

No, they are in different cases! Læknir (doctor) is the subject, so it is in the nominative case. Its nominative definite ending is -inn. Handleggur (arm) is the object, so it is in the accusative case. For many masculine words, the 'the' suffix looks identical in these two cases even though the end of the base word might change.

How does this sentence relate to our current lesson on the genitive case?

While there are no genitive words in this specific sentence, we can easily create one! If you wanted to say 'the arm of the doctor' (the doctor's arm), you would use the masculine genitive definite suffix -sins on the owner. The phrase becomes handleggur læknisins.

How do we know lagaði is in the past tense?

The infinitive verb is að laga (to fix). It is a regular weak verb, meaning it forms its past tense by adding an ending with a dental letter like ð, d, or t. Here, it takes the very common -aði ending for the third-person singular (he/she/it fixed).