Were you a bird, you ___________________ in the sky.

Concept: A present unreal (second / hypothetical) conditional describes an imagined or contrary-to-fact situation in the present. Its structure is: if-clause in the past simple, and a result clause using "would" + base verb. The if-clause can be made more formal by inversion, dropping "if" and putting the verb first: "Were you ..." means exactly "If you were ...".

Application: Here the if-clause is the inverted "Were you a bird" (= "If you were a bird"), an imagined situation that is not real. The blank is the result clause, so it must take the form "would" + base verb:

1. Identify the if-clause: "Were you a bird" is an inverted unreal condition in the present.

2. Apply the rule: the result clause of a present unreal conditional uses "would" + base verb.

3. Fill the result clause: "would fly" gives "Were you a bird, you would fly in the sky."

Contrast: the other choices carry the wrong modal meaning for an unreal present situation:

• "shall fly" — "shall" marks a real simple future or formal intention, not an imagined / contrary-to-fact result.

• "should fly" — "should" carries obligation, advice, or expectation, not the hypothetical result of an unreal condition.

• "shall have flown" — a future-perfect form for an action completed in the future; it cannot pair with a present unreal condition.