1 “Period” 是名词，它的意思包括 “一段时间，时期”。表达这个意思时，常见搭配有：over/during a period（在一段时间内）；a period of time（一段时间）。

We'll observe the patient's weight for a period of three months.

During this period, we normally get a lot of rain.

2 “Period” 还可以表示 “（人生中或历史上的）阶段，时期”。表达这个意思时，“period”  前通常有形容词来描述这是 “怎样的” 一段时期。

I went through a difficult period when I returned to the UK.

The Elizabethan period in English literature is famous for its playwrights such as Shakespeare.

3 “Period” 用在谈论学校、上课的上下文中，可以表示 “学时，课时，一节（课）”。

I have maths in the second period on Tuesday.

There are three periods of physical education each week.

4 在美式英语中，“period” 还可以表示 “句号”，对应英式英语中的 “full stop”。

Is that a period or a comma? 

I always forget to end my sentences with a period.