Ég set hvítan borðdúk á borðið.

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Breakdown of Ég set hvítan borðdúk á borðið.

• borðið: the table
• hvítur: white
• setja: to put

Questions & Answers about Ég set hvítan borðdúk á borðið.

Why is hvítan ending in -an?

Because borðdúk(ur) is a masculine noun, and here it’s the direct object of the verb set (to put/set), so it’s in the accusative singular:

• nominative: hvítur borðdúkur
• accusative: hvítan borðdúk
  The adjective has to match the noun in gender, number, and case.

Why is it borðdúk and not borðdúkur?

Borðdúkur is the nominative singular (dictionary form). In the sentence it’s the object, so it takes accusative singular, which for many masculine nouns drops -ur:

• borðdúkur (nom.) → borðdúk (acc.)

Why is it á borðið (accusative) instead of á borðinu (dative)?

With á, Icelandic uses:

• accusative for movement/direction onto something (putting it onto the table): á borðið
• dative for location/being on something (already on the table): á borðinu

So Ég set … á borðið focuses on the action of placing it.