1. I Newton /performed/ many experiments with light, Past Simple
2. When
Isaac /was/ fourteen years old. his mother thought it /was/ time for him to
begin working on the farm as she /wanted/ her son became a farmer.Past Simple
3 Young Isaac Newton /was peacefully sitting/ in the garden of his home
Past Continuous
4 His main discovery /is/ the discovery of the law of gravity. Present Simple
5 Isaac Newton /has solved/ one of the great riddles of the universe Present Perfect
6 Newton /picked/ up the apple which /had struck/ himPast Simple, Past Perfect