一、表示“光线”“亮光”，一般为不可数名词；表示“在……亮光下”，可用介词 in 或 by。其区别是：用 in 通常只客观地表示“在……亮光下”，而用 by 则暗示“凭借亮光做……”之意。如：

They went out for a walk in the moonlight. 他们在月光下到外面散了步。

He worked by the light of the moon. 他在月光下工作（即凭借月光工作）。

The light is too weak to read by. 光线太暗不能看书。

二、表示“灯”，注意搭配：

The light is [goes] out. 灯熄了。

The light is [comes, goes] on. 灯亮了。

He turned [switched, put] on the light. 他打开灯。

He turned [switched, put] off the light. 他关掉灯。

三、表示“火”，通常只指抽烟用的“火”，注意不要与 fire 弄混。如：

能借个火吗?

误：Have you got a fire, please?

误：Can you give me a fire, please?

正：Have you got a light, please?

正：Can you give me a light, please?