Breakdown of Этот нож острый, смотрите, не порежьтесь!
не
not
этот
this [f: эта, n: это]
смотреть
to watch / to look at / make sure (in a warning)
нож
knife
Questions & Answers about Этот нож острый, смотрите, не порежьтесь!
Normally, negative commands ("don't do something") use the imperfective aspect. Why is порежьтесь perfective?
You've hit on the exact exception to the rule! Usually, telling someone not to do an action requires an imperfective verb. However, when you are warning someone about an accidental, undesirable result (like "careful, don't fall!" or "don't cut yourself!"), you use the perfective aspect. The phrase смотрите, не is a classic trigger for these warnings.
The lesson talks about using the Perfective Future for warnings, but isn't порежьтесь an imperative (command) form?
Great eye! Yes, порежьтесь is the perfective imperative. Warnings with смотри, не (or the plural/formal смотрите, не) can actually take either the perfective future (не порежетесь) or the perfective imperative (не порежьтесь). Both mean the exact same thing, but the imperative form used in this sentence is incredibly common in everyday spoken Russian.
Why does порежьтесь have a soft sign (ь) in the middle?
This comes down to spelling rules for the imperative. When a verb's imperative stem ends in a consonant (like пореж-), we add a soft sign before adding the plural/formal ending -те and the reflexive ending -сь. So, the base singular command порежь becomes порежьтесь.
Why is there a -сь at the very end of the verb?