Learn Finnish: Luulin koiraa vaarattomaksi ja annoin sille ruokaa. - I thought the dog was harmless, and I gave it food.

This is the specific structure used with the verb luulla (to think, to assume). When you express what you thought something was, the object takes the partitive case (koiraa) and the description takes the translative case (vaarattomaksi).

Where does the tricky form vaarattomaksi come from?

The base word is vaara (danger). Adding the suffix -ton (without) creates the adjective vaaraton (harmless). When we add the translative ending -ksi, the -ton ending must change to its stem form -ttoma-, which gives us vaarattomaksi.

Why is ruoka in the partitive case (ruokaa) here?

The partitive is used here because food is an uncountable mass noun, so you are giving 'some food' rather than a whole, specific portion. Notice that this differs from koiraa, which is in the partitive purely because the verb luulla demands it.

Why do we use sille instead of sitä or sillä?