Bonjour Dzoan ! This is indeed a complicated case, which shouldn't probably feature in an A2 lesson. But as it's here, I'm going to explain it :) The second part of the sentence is introduced by the expression "jusqu'à ce que" (= until does ) which is always followed by Le Subjonctif (Subjunctive mood). "S'endormir" here is conjugated in Le Subjonctif Présent, hence the confusing ending. Here is the glossary article link about Le Subjonctif: https://kwiziq.learnfrenchwithalexa.com/revision/glossary/verb-tense-mood/french-present-subjunctive I hope it helps! À bientôt !