Learn Russian: Камни здесь очень скользкие, не поскользнись, если пойдёшь один. - The rocks here are very slippery, don't slip if you go alone.

You are right that negative commands telling someone not to do something on purpose use the imperfective aspect. However, when warning someone against an accidental, undesirable action (like slipping, dropping something, or getting sick), Russian uses не plus the perfective imperative. This structure acts as a warning: 'watch out so that you don't accidentally slip!'

Why does the reflexive imperative поскользнись end in -сь instead of -ся?

In Russian, the reflexive particle changes shape depending on the letter immediately preceding it. Because the imperative stem ends in a vowel (поскользни), the shortened particle -сь is attached. If the form ended in a consonant, such as in the infinitive поскользнуться, it would use -ся.

Why use the perfective future пойдёшь instead of the imperfective идёшь in the 'if' clause?

The perfective пойдёшь (from пойти) focuses on the concrete future action of setting off on this specific trip. The speaker is establishing a specific future condition ('if you set off alone later'), rather than talking about an ongoing process of walking, which would use the imperfective.

How would this sentence change if the speaker were warning a female friend?