Bonjour,
Réponse :
1. We were eating in a restaurant in Hollywood when we saw that famous film star.
2. Gizem lost her camera while she was travelling ( GB )/ traveling ( US )) in South America.
3. It was snowing when they arrived in Paris.
4. They stopped in Oxford while they were driving across the UK.
5. He was walking through the museum when he met an old friend.
6. You took a wonderful photo while we were staying in Egypt.
Explications :
Past continuous ( was/were + V-ing ) pour décrire ce qui était en train de se passer ( correspond à l'imparfait )
Past simple pour les actions courtes, qui "interrompent" ( correspond au passé-simple / passé composé )
