Learn Finnish: Lastatkaa kontit nopeasti! - You all load the containers quickly!

It is the second-person plural imperative (command) form of the type 4 verb lastata (to load). You use it when telling two or more people to do something.

How is the imperative plural lastatkaa formed from the verb lastata?

For verb type 4, you take the infinitive lastata, drop the final -a to get the consonant stem lastat-, and then add the plural imperative ending -kaa.

Why is the object kontit (containers) in the nominative plural form?

When telling someone to do a complete action to multiple items, the total object takes the plural nominative form (ending in -t). If it were just one container, it would also be in the nominative singular (kontti) because total objects of imperative verbs lose the genitive -n ending.

The current lesson is about the present passive referative (-tavan). Why isn't it used in this sentence?

The referative construction replaces an että (that) clause in reported speech, like Tiedän kontit lastattavan (I know that the containers are being loaded). This sentence is just a direct command, so it uses the standard imperative mood instead.