用于介绍起因或原因，语气最强烈，表示直接原因，接在主句后面。

I didn’t answer your messages because I was out of the country.
我没有回复你的留言，因为我出国了。

Because my lower back kept hurting, I decided to finally go see a chiropractor.
因为我的后背疼，我最终决定去看脊椎按摩师。

可以用来介绍起因或原因，表示对方已知的无需加以说明的原因或事实，语气比because弱。

I decided to bake cupcakes, since it was Marjorie’s birthday.
因为那是马乔里的生日，所以我决定烤蛋糕。

Since you’re always late, I’m going to start showing up late too.
因为你总是迟到，我也打算晚到了。

也可以用来表示某事从某个时间点开始是真的。

Ever since I was young, I’ve always wanted to become a scientist.
从我年轻的时候起，我就一直想成为一名科学家。

What have you been up to since school ended?
放学后你一直在忙些什么？

可以用来指某事直到某一刻才发生。

I usually sit around in my office until my boss gives me work to do.
我通常坐在办公室里，直到老板给我工作做。

可以用来表示两个事件同时发生。

When it started to snow, everyone started posting statuses on Facebook.
当开始下雪时，每个人都开始在脸书发布状态。

When the clock struck three, all the students immediately evacuated the classroom.
当钟敲到三点时，所有的学生立即撤离教室。

可以用来表示两件事同时发生。相比于连词“when”，它更强调动作的持续性。

I often get distracted while trying to study.
我经常在学习时分心。

It’s hard trying to take classes while also working two jobs.
做两份工作还要同时上课学习是很困难的。

也可以用来表示转折，表示“尽管”。

While puppies are cute, they can be incredibly annoying to take care of.
虽然小狗很可爱，但照顾它们却让人很烦躁。

可以当做“while（表示同时）”的替换词；

As the night drew on, the crowd became noisier and noisier.
当夜幕降临的时候，人群变得越来越吵。

也可用作“since”或“because”的替换词；

You should be careful going to the gym, as your ankle is still a little weak.
你去健身房要小心点儿，因为你的脚踝还有点问题。

也可以用来表示“以...的方式”，可以用“just”这个词来强调。

I wrote my essay with five paragraphs just as my professor told me to do in the instructions.
按照我教授告诉我的提示，我写了五段文章。

Just as you requested, here’s your coffee with soy milk instead of regular milk.
正如你所要求的，这是你的咖啡，用豆奶代替普通牛奶。

可以用来表示假设。

If I lived alone, I’d play my muisc really loudly.
如果我独自一人生活，我将把我的音乐播到最大。

If something’s bothering you, don’t hesitate to tell me.
如果有什么事困扰着你，请告诉我。

连词“if”可以用“even”来强调。

Even if I’m having a bad day, I try to be nice to people.
即使我今天过得不好，我也会对别人友善。