He had begun to feel sleepy.

他已觉得困了。

Remember (that) you may feel sleepy after taking the pills.

记住，吃了这些药片之后，你可能会犯困。

Many students not feel sleepy until much later in the evenings.

许多学生直到晚上很晚才感到困倦。

Go to bed if you feel sleepy.

如果你困了，就睡吧。

Suppose you feel sleepy.

假定你感觉困倦。