Breakdown of На полу стекло, смотри, не порежься!
не
not
на
on, at / by (transport) / to (a place) / to (in answer to)
смотреть
to watch / to look at / make sure (in a warning)
пол
floor
Questions & Answers about На полу стекло, смотри, не порежься!
Why do we use the perfective verb порежься here? Normally, negative commands use imperfective verbs!
You are right that negative commands (telling someone NOT to do something) usually require imperfective verbs. However, there is a major exception for warnings. When warning someone about a negative outcome they might do by accident, we use the perfective aspect.
The current lesson is about using смотри, не with the perfective future. Is порежься in the future tense?
Порежься is actually in the imperative (command) form! When giving warnings with смотри, не, Russians use both the perfective future (like смотри, не порежешься) and the perfective imperative (like смотри, не порежься). Both are perfectly natural ways to say 'watch out, don't accidentally cut yourself.'
Why does the word for floor (пол) end in -у here instead of the usual prepositional ending -е?
Пол belongs to a small group of short, masculine nouns that take a special stressed -у ending in the prepositional case, but only when talking about physical location (answering the question 'where?'). That is why we say на полу (on the floor) instead of на поле.
What is the infinitive of порежься and why is there a soft sign (ь) before the -ся?