Man is born again and again to enjoy or suffer the results of karma left after undergoing experiences such as heaven.
Let us say when a child is born it brings forward a total accumulated karma "A" from the previous births. Part of it will be experienced in the present birth and another part will be kept for the future births.
A = B + C
B = karma to be experienced in the present birth
C = karma to be experienced in the future births.
Onto B are added, D - karma coming through the mother and her family and E - karma coming the father and his family.
B + D + E
Now, there is karma generated in the present birth, either through actions out of one's own free will, or karma generated while exhausting the previous karma. This may be called "F".
If the soul is heavily loaded with intense past karma, it will have no time or chance to exercise its free will. These are the cases where life looks like a straight jacket of fate and destiny. However, in most cases some free will is available to be exercised.
While B, D and E will be fully exhausted in the present life, F will be partly exhausted in the present life and the balance adds to C for the future birth.
G = part of F to be experienced in the present birth
H = part of F to be carried forward to the future birth.
Total karma to be experienced in the present birth = B + D + E + G
Karma carried forward to the future birth = C + H.
This means the present life experiences are the results of two kind of actions
B + D + E coming from the past not under one's control anymore.
G, arising out of one's action in the present birth.
That is why sastra says
Atra tu dvividham karma, prakkarma nijayatnantaha
There are two types of karma, previous karma and one generated here.
So in most cases the experiences are a mix of both, coming from the past births and those generated by one's own action here.
If a boy is not studying well, it could be due to two reasons or their combination.
Vidya pratibandha (obstacle in studies ) due to past karma or lack of effort, improper training, bad environment etc. all pertaining to the present.
It should be carefully ascertained which one. If the horoscope does not show afflictions in education coming from the past then it is clearly due problems other than past karma and should be tackled in the appropriate manner.
Going back to the next birth. the soul carries forward C + H into the next birth. The soul after death remains in a suspended state called preta avastha for some time during which some karma is exhausted but no fresh karma accrued. Further to this the meritorious ones attain pitruloka and stay there until the third generation joins in. Those souls with lot of bad karma can not attain pitruloka unless their load is lightened by prolonged suffering as pretas. The soul can not enter pitruloka with a huge load of bad karma. There is a baggage limit.
No karma is acquired in pitruloka also.
After spending time at pitruloka the soul is put on a fast track to exhaust a large part of its C + H. This takes place at the svarga or naraka through intense experiences of pleasure and pain. When the karma comes down to a level manageable through earthly experiences the soul takes rebirth.