Category F:
Between April 5, 2014 and April 4, 2015, your fiancé will have needed to have made £18,600 BEFORE expenses (the amount he paid tax on).
Category G:
Allows 2 financial years. So between April 5, 2013 and April 4, 2015, your fiancé will have needed to have made £37,200 BEFORE expenses. This means that if he didn't make £18,600 in the prior financial year but made over that the previous financial year, he can use that to meet the requirement.