Learn Swedish: Vänner ska respektera ens hemligheter. - Friends should respect one's secrets.

Ens is the possessive form of the generic pronoun man (one). It translates to "one's". We use it here because the sentence is making a general statement about "a person's" secrets, rather than pointing to a specific "his" or "her" secrets.

What would happen if I used sina instead of ens in this sentence?

It would completely change the meaning! Sina is a reflexive pronoun that points back to the subject. If you wrote Vänner ska respektera sina hemligheter, it would mean "Friends should respect their own secrets." Using ens shows they are respecting someone else's secrets.

I learned that ska means "will". Why is it translated as "should" here?

While ska often marks the future tense ("will" or "am going to"), it is also used to express a strong expectation, a rule, or a moral duty. In these contexts, it translates to "should" or "are supposed to". To sound a bit less strict, you could use borde instead.

Why is hemligheter (secrets) in the indefinite form?