[Grammar] learn to V / Ving?

[sitifan]

1. He learned to dance when he was fifteen.
2. He learned dancing when he was fifteen.
3. He learned to swim when he was fifteen.
4. He learned swimming when he was fifteen.
5. He learned to play soccer when he was fifteen.
6. He learned playing soccer when he was fifteen.

Which of the above sentences is not acceptable?

 

[jutfrank]

The odd-numbered ones are the correct ones.