Learn Portuguese (Portugal): Quem me dera reservar o quarto mais caro do resort. - I wish I could book the most expensive room in the resort.

It is a fixed expression used to say "I wish" or "If only". Literally, it translates to "Who would give to me", where dera is a past tense form of dar (to give). You use it to express a strong desire or longing for something.

Why is reservar in the infinitive here? Doesn't expressing a wish usually require the subjunctive?

Because the person wishing and the person booking the room are the same ("I wish I could book"). When the subject doesn't change, Quem me dera is followed directly by the infinitive. If you wished for someone else to do it, you would use que plus the imperfect subjunctive, like Quem me dera que reservasses (I wish you would book).

How does o quarto mais caro translate to "the most expensive room"?

To say "the most" followed by an adjective in Portuguese, you use the definite article (o or a) + the noun + mais (more) + the adjective. So, o quarto mais caro is literally "the room more expensive".

Why does the sentence use do resort instead of no resort for "in the resort"?