How to use 'once'.

I would like to know the tense after once.  As I know, we need to use past tense after 'once ', but if I am referencing the future event (or not happen event), what tense should I use.
Please kindly provide Some examples will be greatly appreciated. Thanks

Once = 有一次.  Therefore the event had happened.  So it must be in the past tense.  If you are referencing something in the future, then future must be used.

1.  I eat a piece of cake.    - simple present.  食塊蛋糕

2.  I ate a piece of cake     - past tense          食佐塊蛋糕

3.  I had eaten a piece of cake. - past tense with the implication, the cake was eaten.食佐塊蛋糕

4.   I shall eat a piece of cake - future tense.  No indication the cake was eaten.

5.  I will eat a piece of cake.           一定要食塊蛋糕

6.  I must eat a piece of cake.         需要食塊蛋糕

7.  I shall be eating a piece of cake - future continuous 想要食塊蛋糕, 形容食僅