Learn Croatian: Zaključaj bicikl lokotom kada stigneš kući. - Lock the bicycle with the lock when you arrive home.

Lokotom is in the instrumental case. When expressing the tool or means used to perform an action—like locking a bike with a lock—Croatian uses the bare instrumental case. The preposition s (or sa) is strictly reserved for company or association (for example, if you were locking the bike alongside a friend).

Is lokot the standard word for any kind of lock?

Not quite. Lokot specifically refers to a padlock or a detached lock, such as a bicycle chain or a U-lock. A built-in lock, like the one on a front door or a car door, is called a brava.

The English translation refers to the future ('when you arrive'). Why is stigneš in the present tense?

Stigneš is the present tense form of the perfective verb stići (to arrive). In Croatian, perfective verbs cannot be used in the present tense to describe an ongoing action. Instead, they are frequently used in dependent clauses starting with words like kada (when) or ako (if) to indicate a future action that will be fully completed before the main action occurs.

Why do we use kući here instead of kod kuće?

Kući is the dative/locative form of kuća and is used here to express motion or direction towards a destination ('homewards' or 'to the house'). Because the verb stigneš implies arriving at a destination, this directional form is required. Kod kuće strictly indicates a static location ('at home') and cannot be used with verbs of motion.