The inconsistencies drive me bonkers. Sometimes "Turn on the light" is "E hōʻā ʻoe i ke kukui", and sometimes itʻs "E hōʻā i ke kukui." Both are correct but I canʻt tell which one is the right one in each instance.
The word ma is used for location only. The light (kukui) is what is being turned on and does not indicate location. Thus, only the word i can be used before it.