Learn Croatian: Tko stoji pred ogradom? - Who is standing in front of the fence?

The preposition pred (in front of) takes the instrumental case when describing a stationary location. Because the subject is just standing still (stoji), there is no motion towards a destination. If someone were moving to the front of the fence, you would use the accusative case instead.

Does the preposition pred only refer to physical space?

No, pred is also commonly used for time, meaning 'before' (for example, pred večeru - before dinner). Keep in mind that when used for time, it takes the accusative case rather than the instrumental case used here for a fixed location.

Why does tko trigger the singular verb stoji if we don't know how many people are there?

The question word tko (who) always acts as a third-person singular subject. Just like in English (where we say 'who is' and not 'who are'), you use the singular verb stoji even if you suspect a whole group of people is standing there.

What is the base form of ogradom and how does its ending change?

The dictionary form is ograda (fence), which is a feminine noun. To form the instrumental case, standard feminine nouns ending in -a drop that vowel and add -om.