Learn Portuguese (Portugal): Tem cuidado com esse caixote desajeitado, que vais riscar o chão! - Be careful with that awkward box, because you are going to scratch the floor!

This is the explicative que (also called causal que). In everyday spoken Portuguese, it is very common to use que instead of porque to give a quick, immediate justification, especially right after a command or strong warning like Tem cuidado. In English, it translates naturally as "because" or "or else".

The translation implies "you" (informal), but the verb is tem instead of tens. Why?

Tem is the affirmative imperative (command) form for tu. To form the affirmative imperative for tu, you drop the -s from the present tense form (tu tens becomes tem). If you were giving this warning formally to a você, it would be tenha cuidado.

Why use esse instead of aquele for "that" box?

Esse is specifically used for something near the listener. Since the speaker is telling the person to be careful with the box they are currently holding or standing right next to, esse is the only natural choice. If the box were far away from both the speaker and the listener, they would use aquele.

Does desajeitado usually mean "clumsy" when talking about a person?