Question

Откройте скобки в Past Simple или в Past Perfect Suddenly he (to remember) that he (not to ring) her up in the morning/

Answer 1

Suddenly he remembered that he had not rung her up in the morning

Answer 2

по-моему, RING - неправильный глагол.

ring - rang - rung

поэтому иоя версия такая:

 

He remembered that he hadn't rung her up in the morning.