This is the 6243rd most frequent German word.
dießen
"Dießen" is not a standalone German word; it may be a proper noun, such as the name of a town (e.g., Dießen am Ammersee).
In this sentence, 'diesen' is used as the accusative demonstrative pronoun referring to 'Stift', modifying the noun 'pen' to be understood as a specific item.
Ich kann diesen Stift nicht finden.
I cannot find this pen.
'Diesen' acts as the accusative demonstrative pronoun modifying 'Sommer', specifying the summer being referred to.
Wir waren diesen Sommer am Meer.
We were at the sea this summer.
'Diesen' is used as the accusative demonstrative pronoun to point out the specific 'Hund', indicating which dog was purchased.