SUMIF and subtraction of specific cells issue...request for help :)

Hi everyone
Very new to excel formulas, so i apologize in advance...I have searched online and found similar formulas but i havent tweaked them to make it do what i want....the more involved it gets the less i understand.

situation 1:
D12 E12 F12 G12
250 252 2 -2

these are pounds lost or gained (weight) so D12 is the starting weight, E12 shows a gain of 2 so i only want a number to appear in F12 if the number goes up (above 0) so F12 would reflect a 2 LB weight gain. I dont want anything to show in G12. It currently does because all i could do is have each cell set up separately to calculate D12 and E12 the way i want.

So ideally it would look like this if the E12 cell is higher than D12: (i suppose if its easier G12 could show a 0 but i want a 0 or nothing at all if possible)
D12 E12 F12 G12
250 252 2

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situation 2:
D12 E12 F12 G12
250 248 -2 2

In this one I want the opposite. When the weight is lost I want to subtract E12 from D12 and the answer to only show in G12 and nothing in F12

So when D12 cell is higher I would like it to look like this:
D12 E12 F12 G12
250 248 2

i hope that makes sense and I would really appreciate any help on creating the formula to make that work....i am really not experienced in it so I can not seem to create it myself just searching online...i always have an error.

thanks for any help!

Thank you for the quick reply...will this work in open office? do they function the same for formulas? I am on a different PC for a couple days so I cant test that yet but those suggestions do not work on my openoffice version....should i be putting something in between the quotations? forgive me as my experience is minimal to nil....only what i find on the web when i search. I appreciate the help though very much!