This is the 197th most frequent Icelandic word.
tveimur
The Icelandic word "tveimur" means "two" in the dative case.
Here, 'tveimur' is the dative plural form of the number 'two,' and it is used because the verb 'keyri á' governs the dative case.
Ég á tvo bíla en keyri aðeins á tveimur.
I own two cars but drive only two of them.
In this example, 'tveimur' is again in the dative plural, used because of the preposition 'í' when indicating division or equal distribution.
Við ætlum að skipta kökunni í tveimur jöfnum hlutum.
We are going to divide the cake into two equal parts.
In this case, 'tveimur' is used in the dative case, which is required by the verb 'gaf' when specifying to whom the object (book) is given.