Questions & Answers about Mies istui vaunussa kädet taskussa.
Why is there no word for 'with' in kädet taskussa?
This is an absolute phrase of state. In Finnish, you can express 'with [body part] in [position]' simply by putting the body part in the nominative case (kädet) followed by a local case or adverb (taskussa). No word for 'with' is needed.
Why is taskussa (in pocket) singular when the man clearly has two hands and two pockets?
In Finnish, it is customary to use the singular form for items of clothing or locations associated with paired body parts, as long as each part goes to its own respective place. So, kädet taskussa literally translates to 'hands in pocket', but it naturally implies that each hand is in its own pocket.
Could I add possessive suffixes and say kätensä taskussaan?
You could, and it would be grammatically correct, but it is not necessary. In these absolute phrases of state, it is already obvious that the subject is acting upon their own body, so the possessive suffixes are usually left out for a more natural, idiomatic flow.
What kind of carriage does vaunu refer to here?
Depending on the wider context, vaunu usually refers to a train carriage, a horse-drawn carriage, or even a baby pram (though that is usually plural: lastenvaunut). It is not used for a normal passenger car, which would simply be auto.