If I ___ (be) a bird, I ___ (fly) across the world.

The correct way to fill in the sentence is: "If I were a bird, I would fly across the world."

The reason for using "were" instead of "was" is because this sentence is a hypothetical situation. In English, the subjunctive mood is used for situations that are contrary to fact or unlikely. With the verb "to be," we use "were" for all subjects in such situations, especially in formal contexts.

The second part of the sentence uses "would fly" to indicate the conditional action that would occur if the hypothetical situation were true. In this case, it's expressing a wish or an imagined scenario where the speaker is a bird and has the ability to fly across the world.

So the complete sentence reflects a conditional statement about an unreal situation.