Learn Croatian: Vidim svaki pokret. - I see every movement.

Svaki (every) behaves like an adjective, so it must match the noun pokret (movement) in gender, number, and case. Because they are the direct object of vidim (I see), they take the accusative case. However, for masculine, inanimate (non-living) objects, the accusative form looks exactly like the dictionary form. So, svaki pokret stays exactly the same.

If I wanted to say "I see every boy", would I still use svaki?

No, you wouldn't. The rule for masculine objects in the accusative case changes if the noun is alive (animate). Because a boy (dječak) is animate, you must use the animate accusative ending -og for the determiner. You would say vidim svakog dječaka.

Could I use sve instead of svaki in this sentence?

Sve means "all", while svaki means "every" or "each". If you wanted to say "I see all movements", you would use the plural form: vidim sve pokrete. This sentence uses svaki pokret to emphasize seeing each individual movement.

Is the word ja (I) missing before vidim?