Its a healthy revision for me,so thanks for the question firstly.
Simple - Eating apples quickly. - Eating is more like a verb.
Complex - The quick Eating of apples. - More of a noun/subject like.
I enjoyed drinking the water and the wine tasting. - Drinking - Simple; Wine tasting - COmplex.
I enjoyed drinking the water and tasting the wine - drinking and tasting - both simple.
Lesson learnt - Simple and Complex gerunds are never parallel.
I believe this will help.
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