Learn Finnish: Näemme lastia siirrettävän nosturilla. - We see that the cargo is being moved with a crane.

This is the present passive referative construction. It replaces an että (that) clause. Instead of saying ...että lastia siirretään (that the cargo is being moved), we take the passive participle siirrettävä and add the -n ending to get siirrettävän. The main verb näemme (we see) is one of the verbs of perception or thinking that triggers this construction.

Can I just use an että clause to say this instead?

Yes! In everyday spoken Finnish, it is actually much more common to use an että clause: Näemme, että lastia siirretään. The -tavan/-tävän construction you see in this sentence is more compact and is used heavily in written or slightly more formal Finnish.

Why is the object lastia in the partitive case?

When you use a referative construction, the object's case follows the rules of the action happening inside it. Since the action of moving is ongoing and incomplete—we are watching it happen right now—the object lasti (cargo) takes the partitive case (lastia).

Why does nosturilla have the -lla ending?

The adessive ending -lla (or -llä) is often used to express the instrument or tool used to perform an action. Here, it simply means "with a crane" or "by using a crane."