If you want to order your coffee the Singaporeans way when you visit a traditional coffee shop:
Kopi O - Coffee with sugar
Kopi - Coffee with condensed milk
Kopi C - Coffee with additional of evaporated milk
Kopi Siew Tai - Coffee less sweet
Kopi Ga Tai - Coffee more sweet
"Kopi" are malay word which means "Coffee". Because Hainanese are the pioneer in selling traditional coffee in Singapore, words "siew tai" & "ga tai" are Hainanese language which are still in use until today.
The operators will also understand if you order the normal way too.