Learn Finnish: Hän manipuloi potilasta ja saa hänet kertomaan totuuden. - She manipulates the patient and gets him to tell the truth.

This is the causation structure you are currently learning. When you use saada to mean causing someone to do something, the person is the object (hänet) and the action they perform must be in the 3rd infinitive illative form, which ends in -maan or -mään. So, saa hänet kertomaan translates to gets him to tell.

Why is the first object in the partitive (potilasta) but the second object is accusative (hänet)?

This perfectly highlights how Finnish uses cases to show the result of an action. The verb manipuloida takes a partitive object (potilasta) because manipulating is seen as an ongoing, open-ended process. However, the causing action with saada was entirely successful and complete—she actually got him to tell the truth—so its object takes the accusative form. Remember that personal pronouns like hän have a distinct accusative ending in -t.

How do I know saada means to get someone to do something here, rather than to receive or to be allowed to?

The clue is the sentence structure, or rection. If you see saada followed directly by an object and a verb in the -maan/-mään form, it almost always carries this causative meaning of making, convincing, or getting someone to do that action.

Why is totuuden in the genitive-accusative case ending in -n?