learn后面接什么？

Learn to ride a bicycle

如果你强行要把to去掉的话就应该是
Learn riding a bicycle
不过一般没必要这样。

learn to do/doing sth.

学习骑自行车 learn to ride a bike./Learn riding a bicycle .

learn to do sth.

学习骑自行车 learn to ride a bike

learn to ride bike

Learn to ride a bike