Language/French/Grammar/"J'ai-eu"-or-"j'avais"
It is complicated indeed. French speakers have the same problem in the other way!
It's all about the duration : you can imagine the past time as a long line. On this line, sometimes events happen ; or you do some actions ; you hear something ; etc.
On the line :
- Let's say there is always a "general context" : you were someone / you had some occupation / you went around with someone... somewhere in the past (for instance : two years ago and for some period) : that was the general context of your life then -> IMPARFAIT (j'avais)
- However there also appear some specific and momentary events on the line. Some which will be marked as crosses or so... For instance : you won a contest (at a specific date) ; you succeedeed in your exams (a specific year) ; you had an accident ; you met someone (specific time and place...) -> PASSÉ COMPOSÉ (j'ai eu)
* * En 2013, je n'avais encore que deux enfants. J'ai eu mon troisième enfant en 2014. (Mon troisième enfant est né en 2014.) J'ai trois enfants.
** En 2012, j'étais étudiante. J'ai obtenu mon diplôme au mois de juin 2012.
Here you can make the comparison present perfect / passé composé but be careful.. because it's rare!! I mean that you can say :
"J'ai étudié l'histoire." (this applies that this experience belongs to the being you are right now... quite like present perfect so)
But this sentence is also correct : "J'étudiais l'histoire [cette année-là]"... (you just talk about you past occupation... as a past one)
Attention : all of this is the same when you talk about a very close past. It should be no problem if you keep in mind the same image of a regular line where specific events appear...
-> Hier, j'étais au restaurant quand tu m'as téléphoné.
Ce matin, j'étais encore au lit quand le facteur a sonné.
It's never about the length between the event and now ; always about the nature of the activity ---or sometimes about what we want to tell about it (do I want to say that I used to study something some years ago ; or that I have studied something and then I know things about it now...)---.
Il y a une heure, j'étais au travail.
Il y a une heure, je me suis endormi au travail.
-> "Il y a une heure" doesn't matter itself.
I hope this will help!