Learn Russian: Никто не должен плавать в этой реке, не то утонет. - Nobody should swim in this river, or they will drown.

не то acts as a conjunction meaning 'otherwise' or 'or else,' specifically introducing a negative consequence or warning. You could absolutely use а то here—they are completely interchangeable in this context for expressing threats or warnings.

Why is утонет in the singular form when the English translation says 'they will drown'?

In Russian, the implied subject of утонет refers back to Никто (nobody). Никто is grammatically singular and requires singular verbs. While English often uses the singular 'they' to refer back to 'nobody', Russian strictly keeps the third-person singular form to match.

Why use the multidirectional verb плавать instead of the unidirectional плыть?

плавать is used here because the warning is about swimming in general, not making a specific trip from point A to point B. Additionally, after negative expressions of obligation like не должен (should not), Russian typically requires the imperfective aspect to express a general prohibition.

Is Никто не должен considered a double negative?

Yes. Russian requires negative concord, which means multiple negatives are used together to form a single negative meaning. Whenever you use a negative pronoun like Никто (nobody), you must also negate the verb with не.