"Who did you lose against last Saturday?"
Translation:Contre qui avez-vous perdu samedi dernier ?
The English sentence is grammatically awkward, though common. The sentence should be "Against whom did you lose last Saturday?" The person to whom you have lost is the OBJECT of the preposition "to" Whom should be used to refer to the object of a verb or preposition. When in doubt, try this simple trick: If you can replace the word with “he”' or “'she,” use who. If you can replace it with “him” or “her,” use whom. Who should be used to refer to the subject of a sentence. Since you can't say "They did you lost againt"