Learn Polish: Obie studentki przegadały cały wykład. - Both female students talked through the whole lecture.

The prefix prze- often shows that an action lasted continuously through a specific period of time. By changing the imperfective gadać (to talk or chat) into the perfective przegadać, it means the students successfully spent the entire duration of an event talking.

Why is the time expression cały wykład in the accusative case?

When you use a verb with the duration prefix prze-, the time period you spend doing the action acts as the direct object of the verb. Because of this, cały wykład takes the accusative case, just as if it were a physical object they were acting upon.

Why do we use obie for "both" in this sentence instead of obaj or oboje?

Obie is the specific form of "both" used for feminine plural nouns. Since studentki (female students) is plural and feminine, we must use obie. For male students it would be obaj studenci, and for a mixed-gender group it would be oboje studentów.

Why does the verb end in -ły (przegadały) instead of -li (przegadali)?