I'm not sure what part of this statement you aren't getting, this is the third time I have said it :
Remove Behaviour Description and change BehaviourID to a combobox
I don't know what you mean about connecting the behaviour ID's.
I don't know what blank lines you are talking about.
You need to link the subform to the main form using the Assessment ID.
You can't have the same table as the record source for both the main form and the subform. As stated above, if you use a subform then the main form won't have a record source.