Let's start here:
Over the years, he became more and more knowledgeable.
The main part of the sentence he became more and more bold describes a gradual change. The modifier more and more helps to express this meaning. The time phrase Over the years expresses the period in which this gradual change took place.
It isn't easy to use the preposition over in this kind of time phrase because we don't usually use a period phrase (two minutes, ten weeks, twenty years) after it. Instead, we use a noun phrase that often begins with the, and sometimes with the course of + period phrase:
Over the past two years, my English has improved considerably.
Over the course of my life, I've become much more confident as a person.
Over the next thirty minutes, you will learn how to use today's vocabulary.
Focus on using the preposition over before you move on to the other two prepositions, which have different meanings and uses.