Converting Excel nested If/and statements to Power Query

Hi, I'm new to Power Queries and am having trouble converting a formula to work in Power Query.  I understand I use if/and/then/else, however I have created custom columns in my power query where a lot of the values are null and I'm getting error messages.  My formula in excel is:  =IF(AND($G28<="31/12/2018"*1,$H28=""),$K28,IF(AND($G28<="31/12/2018"*1,$H28<="31/12/2018"*1),$B28,IF(AND($G28<="31/12/2018"*1,$H28>"31/12/2018"*1),$K28,0))).   Any advice would be helpful.  Thanks.