To learn a language we must follow the principle of gradual improvement.

学习语言必须遵循循序渐进的原则。

Main thing: gradual improvement.

最重要的是：要逐渐进步。

His health showed gradual improvement.

他的健康逐渐恢复。

There is a gradual improvement in his work.

他的工作逐步有了改进。

They expect a gradual improvement in sales of new cars.

他们预期新车的销售量会逐步提升。