Phrasal verbs are very common in English but they also cause a few problems.
First, there are very, very many phrasal verbs and it can be hard to know where to start.
Second, phrasal verbs often have more than one meaning. 'Take off' can mean both 'leave the ground' and 'become successful'. 'Go on' has eight meanings in the Oxford Learner's Dictionary. How can we choose which meanings to study?
Third, phrasal verbs are often used only in very specific situations. They have narrow meanings and you will often see the same examples again and again. These are the situations that it's important to learn. Even though 'go on' means 'happen', we can use 'happen' in a lot more situations than we can use 'go on'.