SML foldl function - addition on a list with a condition

Question

I have a university course about functional programming, where I use SML. As a preparation for the exam, I am working on some of the older exam sets without solutions.

One of the only questions I really have problems with is the following question using foldl:

Consider the program skeleton: fun addGt k xs = List.foldl (...) ... xs; Fill in the two missing pieces (represented by the dots ...), so that addGt k xs is the sum of those elements in xs, which are greater than k. For example, addGt 4 [1, 5, 2, 7, 4, 8] = 5 + 7 + 8 = 20

I am sure this is really easy, but I have a very hard time understanding the foldl and foldr functions.

What I have now is the following (which seems to be very wrong if you ask my compiler!):

fun addGt(k,xs) = List.foldl ( fn x => if x > k then op+ else 0) 0 xs;

I would really appreciate some help with this question, and maybe a very short comment which would cast some light on the foldl and foldr functions!

Thanks a lot.