Questions & Answers about Hän luuli tätä miestä huijariksi.
Why is huijariksi in the translative case here?
The verb luulla (to think or assume) triggers this case. When you assume someone is something, the thing they are assumed to be takes the translative ending -ksi. You can think of it as meaning he mistook this man 'for a swindler'.
Why is the object tätä miestä in the partitive case?
In this specific opinion-expressing structure, the person or thing you are making an assumption about must be in the partitive case. That is why tämä mies changes to tätä miestä.
Could I use the verb pitää instead of luulla?
You could, but it changes the meaning. Luulla usually implies a guess, assumption, or a mistaken belief. Pitää means holding a firm opinion or considering someone as something. Importantly, if you use pitää, the grammar changes: you must use the essive case ending -na (huijarina) instead of the translative -ksi.
Where is the word for 'was' in the Finnish sentence?
Finnish doesn't need a past tense form of olla (to be) for this structure. The combination of the verb luulla alongside the translative ending -ksi already carries the full meaning of 'thought someone was something'.