Learn Icelandic: Við sitjum á kaffihúsinu með honum.

The preposition með meaning "with" (when you are accompanying someone) requires the pronoun to be in the dative case. Honum is the dative form of hann (he/him).

Why does kaffihús have -inu at the end in this sentence?

Kaffihús is a neuter noun. The -inu ending is the attached word for "the" in the dative case. It takes the dative case here because the preposition á triggers the dative when describing a stationary location, like sitting at a café.

I thought the preposition á meant "on". Why is it translated as "at"?

Prepositions rarely translate directly between languages! While á frequently means "on", Icelandic naturally uses á for many public places or businesses like cafés, restaurants, and museums. In English, we say "at the café", but in Icelandic, you are technically "on" it!

How do I know the dictionary form of the verb sitjum?

The infinitive (dictionary form) is sitja (to sit). In Icelandic, verbs paired with the pronoun við (we) are very easy to spot because they almost always end in -um.