1 When I arrived, ... she cooked dinner. she was cooking dinner. she'd cooked dinner.

1. When I arrived, ...

she cooked dinner.

This implies that the dinner was finished by the time I arrived.

she was cooking dinner.

This implies that the dinner was in progress when I arrived.

she'd cooked dinner.

This implies that the dinner was finished before I arrived. This is similar to 'she cooked dinner' but emphasizes the completion of the action.