Learn Icelandic: Við brjótum steininn með bornum. - We break the stone with the drill.

The preposition með takes the dative case when it means "with" in the sense of using a tool or instrument, like the drill in this sentence. If it meant "carrying" or "bringing" something along, it would take the accusative case.

Why is the dative definite form of bor (drill) bornum and not borinum?

Many masculine nouns add an -i in the dative singular, but the noun bor does not; its dative singular form is simply bor. When you attach the dative masculine definite article (-num), it connects directly to the stem, giving us bornum.

Why is steininn in the accusative case?

The verb brjóta (to break) requires its direct object (the thing being broken) to be in the accusative case. Steinn is a masculine noun, so its accusative singular form is stein, and adding the definite article (-inn) gives us steininn.

What is the infinitive of brjótum and how does it behave in the past tense?

The infinitive is brjóta (to break). It is a strong verb, which means it changes its internal vowel in the past tense rather than taking a regular ending. At this level, you will start seeing its past forms like braut (broke) and the past participle brotið (broken).