1. Instead of "what do you think when it happened", say "What DID you think when it happened." (if you're going to speak of the past tense, "do" changes to "did") 2. "He wondered if I would go to London in the summer." is correct. (\(`u` )/)~

It should be: 'When do you think it happened?' He wondered if I would go to London in the summer. This should be used if you yourself are not sure if you will go. Otherwise if you are sure about going it must be: 'He wondered if I will go to London in the summer'