Learn Portuguese (Portugal): Não consegues ver a constelação a menos que uses o telescópio. - You cannot see the constellation unless you use the telescope.

The word uses is the present subjunctive form of usar for tu (you). The phrase a menos que (unless) is a trigger that always requires the verb that follows it to be in the subjunctive mood. Since usar is an -ar verb, it swaps its usual 'a' for an 'e' in the present subjunctive, making it uses.

Could I use a não ser que in this sentence instead?

Yes, absolutely! The phrases a menos que and a não ser que both mean 'unless' and are completely interchangeable. Both of them trigger the subjunctive, so the sentence would mean exactly the same thing: Não consegues ver a constelação a não ser que uses o telescópio.

Why use consegues ver instead of podes ver for 'you cannot see'?

While both translate to 'can' in English, Portuguese makes a clear distinction between these verbs. The verb poder is usually for permission or general possibility, while conseguir is used for having the physical ability or managing to do something successfully. Here, seeing a constellation requires physical capability and the right equipment, making consegues the more natural choice.

Why isn't there a subject pronoun like tu in either half of this sentence?