Learn Polish: My byśmy chętnie polecieli na bezludną wyspę. - We would gladly fly to a deserted island.

In Polish, the conditional particle by (along with its personal endings like -śmy) is mobile in main clauses. While it typically attaches to the end of the verb to form polecielibyśmy, it can detach and attach to the first emphasized word in the sentence. Here, attaching it to the pronoun forms My byśmy, which places a slight emphasis on the fact that we (as opposed to someone else) would gladly do this.

If I drop the optional pronoun My, where does the byśmy particle go?

If you remove My, the particle needs a new host. You could attach it back to the verb (Chętnie polecielibyśmy na bezludną wyspę) or, very naturally in conversational Polish, attach it to the adverb at the start of the clause (Chętnie byśmy polecieli na bezludną wyspę).

Why is the past tense form polecieli used when talking about a hypothetical future trip?

The Polish conditional mood is always constructed using the third-person past tense form of the verb (here, the virile/masculine personal polecieli) plus the by particle. Even when the particle detaches and moves elsewhere in the sentence, the main verb must remain in its past tense form.

Why do we use the preposition na instead of do for flying "to" this destination?