Learn Finnish: Pomo jätti työkalunsa vajaan. - The boss left his tools in the shed.

In Finnish, when a third-person subject (here, pomo) owns the object they are acting upon, we drop the genitive pronoun (hänen) and only use the possessive suffix (-nsa). If you included hänen, the sentence would mean the boss left someone else's tools in the shed.

The translation says "tools" (plural), but työkalunsa doesn't have a plural marker. How can we tell?

We actually can't tell just from this word! When you add a possessive suffix like -nsa, the normal object plural marker -t and the singular object marker -n both drop off. Because of this rule, työkalunsa can mean either "his tool" or "his tools". You have to rely on the context.

Why is "in the shed" translated as vajaan (into the shed) instead of vajassa (in the shed)?

The verb jättää (to leave) is treated as a directional verb in Finnish. You aren't just leaving something where it is; you are leaving it into a place. Therefore, it requires a destination case like the illative (-an/-en) or allative (-lle) rather than a static location case.