Learn French: On va chanter un beau duo, ne serait-ce que le refrain. - We are going to sing a beautiful duet, if only the chorus.

While seulement states a simple fact, ne serait-ce que (literally "would it not be but") adds a hypothetical or restrictive nuance. It translates to "if only" or "even just," emphasizing that singing the chorus is the absolute minimum acceptable condition.

Does the tense of serait change if the main sentence is in a different tense?

No, ne serait-ce que is a completely invariable, fixed expression. Even if the rest of the sentence were in the past or present, you would still use the exact same phrase without changing the verb tense or conjugating it.

What kind of words can immediately follow ne serait-ce que?

It is very flexible! In this sentence, it is followed directly by a noun (le refrain). You will also frequently see it followed by a pronoun, or by an infinitive verb (usually introduced by pour, such as ne serait-ce que pour chanter).

Is duo the exact same word as the English "duo"?

It can be, but it has a broader meaning in French. It can refer to two people working together (a duo), but in a musical context like this one, it means a piece of music sung or played by two people (a duet).

Why does the adjective beau come before the noun ?