This is the 207th most frequent Icelandic word.
þennan
The Icelandic word "þennan" means "this" (masculine singular accusative form).
"Þennan" is used as the accusative masculine form of the demonstrative pronoun "þessi" to agree with "hatt" (hat), the object of the sentence.
Ég sá manninn með þennan hatt.
I saw the man with this hat.
"Þennan" is again at accusative masculine form to agree with "bíl" (car) as it refers to a specific object chosen.
Hún valdi þennan bíl fyrir ferðalagið.
She chose this car for the trip.
Here, "þennan" functions as the masculine accusative form of "þessi" used with "bók" (book), showing selection of a specific item.