當我唱歌，就會跳舞。的英文是?

The best is While I'm singing, i will dancing or I will dancing while i am singing. Use While is better than When.
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please note that after [will] you have to use [dance]

當我唱歌，就會跳舞
I will dance while I am singing

I will dance when I am singing.

Once I sing, I begin to dance

Whenever I sing, I dance.

I dance while I am singing.

when I sing, I will dance too.

I dance when I am singing.

When I sing, I will dance.

I will dance when I sing.

When I sing, I will dance.