päiväkirjaan
Päiväkirjaan is the illative singular form of the Finnish noun päiväkirja, meaning a diary. The illative expresses movement toward something or entry into it, so päiväkirjaan is used when referring to writing into the diary or recording information there, rather than simply being inside the diary.
In Finnish, päiväkirjaan is commonly used with verbs of recording or writing, such as kirjoittaa (to write),
Päiväkirjaan contrasts with the inessive form päiväkirjassa, which would indicate that something is located inside the
Etymology traces päiväkirja to päivi + kirja, combining day and book to denote a personal journal kept
Overall, päiväkirjaan is a standard grammatical form used to express writing or recording content into a diary.