Learn Finnish: Satuin tapaamaan entisen pomoni kadulla. - I happened to meet my former boss on the street.

The verb sattua (when meaning "to happen to do something") requires the following action verb to be in the -maan/-mään form, also known as the third infinitive illative. You take the strong stem of tavata (tapaa-) and add -maan.

Since the main verb is satuin, why isn't the object in the partitive case (entistä pomoani)?

While ongoing actions or negative sentences require the partitive, this is a completed, sudden encounter. The verb tavata (to meet) takes a total object here to show the meeting was fully accomplished, so the object is in the genitive-accusative form.

Why does the adjective entisen have an -n at the end, but pomoni doesn't?

The object phrase would normally be entisen pomon. However, when you add a possessive suffix like -ni (my) to a noun ending in the genitive -n, the suffix replaces the -n, making it pomoni. The modifying adjective entisen (former) doesn't take the possessive suffix, so it keeps its -n ending.

Why do we use the adessive case kadulla for "on the street" instead of the inessive?