i have a column called Loaned, dropdown (yes or no). If the user uses 'yes', then there's a "Return date" field below which will require them to put in a date. Is there a way to validate that if they choose Yes in the Loaned field, they are required to put a date in the "Return Date", so that when they hit "save", if no date is there, it will not allow them to save.
Yes, it is possible but not OOTB. Need to put one script editor on list new form and edit form.
There is one method presaveaction write your logic into it and return false if your condition not satisfied and return true if your condition satisfied.
Please check below link for know more about presaveaction method.
Not out of the box. You will need to develop a custom form for that, using a aspx form in sharepoint designer or infopath. Check these out for creating custom new/edit form:
You can build this functionality using the OOTB "Validation Settings" on your list or library. List validation allows you to check multiple fields and combination. Only downside is that you can only have on message for all your list validations. Here is a nice video with explanation of column and list validations: https://www.youtube.com/watch?v=zm5vnSNESPI (after 3:15 about list validation settings).