J'ai eu vs J'avais
I am confused about the following two phases
Are there any subtle differences between the two?
"J'avais", and more generally the imparfait, is used for a past situation which lasted some time, a description. For example: "J'avais une voiture." => "I had a car." You may not have one anymore, but it was your situation at the time.
"J'ai eu", and more generally passé composé, is used mostly when speaking, to refer to an action in the past, that did not last very long. For example: "J'ai eu un accident" => "I had an accident." It was just a moment.
There is a subtlety though: the imparfait can also be used for a temporary situation in the past, that led to an action. For example: "J'avais une voiture quand j'ai eu un accident". => "I had a car when I had an accident." The situation (you had a car) led to, or was true when, the action happened (the accident).
I hope this makes sense! Good luck with French!