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Ответ:

1. Luisa was walking in the park when the rain started.

The boss entered the room while Kate was writing an email.

Emma fell down while she was riding a bike.

The kids were swimming in the pool when their mom called them.

Mark was dancing when I took a photo of him.

Paul was playing the piano while his friends were singing.

Everybody was sitting outside when Jim arrived.

Harry was working on the computer when he received the message.

Объяснение:

Если присмотреться, можно заметить что после

~ when обычно идут verbs in the past simple ( глаголы в прошедшем времени. Например: she + startED, tOOK, arrivED...)

~а после while идут verbs in past continuous (глаголы в прошлом продолжающем времени:

she + WAS swimmING, dancING, singING).

Советую юзать эту логику.