Когда пишется did а когда didn`t

Вспомогательный глагол did используется во временной форме The Past Simple в   вопросительных предложениях, а didn`t - в отрицательных. 

Did he visit his grandparents? 

Did you help your friend with his project?

She didn`t see him last week. 

Maria didn`t buy a new dress yesterday.