Learn Icelandic: Tillitslaus samferðamaður gefur manni ekkert pláss. - An inconsiderate fellow traveler gives one no space.

The verb gefa (to give) dictates the case of its objects. It takes a dative indirect object (the person receiving) and an accusative direct object (the thing given). Since 'one' is receiving the space, the impersonal pronoun maður must take its dative form, manni.

Is the impersonal pronoun maður declined exactly like the regular noun maður?

Yes! The impersonal pronoun (meaning 'one' or a generic 'you') shares the exact same declension as the noun for 'man' or 'person'. Its forms are maður (nominative), mann (accusative), manni (dative), and manns (genitive).

How is the adjective tillitslaus constructed?

It combines the noun tillit (consideration or regard) and the suffix -laus (meaning '-less'). It literally means 'consideration-less'. Notice that an 's' is added to form tillits- because the genitive case is often used to link a noun to a suffix in Icelandic.

What are the parts of the compound noun samferðamaður?

It is built from the prefix sam- (together or co-), the plural genitive of ferð (journey or travel) which is ferða, and maður (person). Literally, it translates to 'a person of joint travels'.