How do I populate a Subform with data that already exists from a lookup field in the main form

How do I populate a Subform with data that already exists from a lookup field in the main form

I have searched the help options for information about this, but can find no example that fits what I'm trying to do. I have a main form with several subforms for which I need different reports (already set up). Each of these reports needs to be sorted by the same fields which recur throughout, Client, Course, Provider and Trainee (all have their own forms). How do I avoid having to enter this information separately in each subform? It all appears as lookup fields on the Trainee form where the subforms also are. 

I've looked at fetching records and think that this is what I need, but the instructions I've found are confusing to say the least. I've never written script before so don't fully understand the terminology. Is there a help section that I'm missing that explains this? Please help. I'm tying myself in knots and am terrified of doing serious damage to what I have already built. And yes I have made a copy of my application, but none of the data came with it so it's difficult to test what I'm doing in there. Thanks

Mel