白 (bái) doesn't only mean "white." When added before a verb it
means to do that verb "in vain" or "with no effect." In a certain
way, you can think of 白 as not being just "white" but also "blank,"
like when you do something for a long time without thinking, and
you're just going through the motions. It's like doing something
with a "blank" mind.
Structure with just 白
All you have to do is but 白 before the verb.One important thing
to note is that 白 is usually used with mono-syllabic words, like 帮,
做, 学 etc. You cannot put a 地 after 白.
Examples
- 白 花 了 一 百 块 钱，这 件 衣服 不 能 穿。I just
wasted a hundred kuai. I can't even wear these clothes.
- 他 不 回来 吃饭 了？！我 这些 菜 白 做了。He's not
coming back to eat？！I made all this food for nothing.
- 你 一 句 汉语 都 不 会 说，白 学 了 这么 长 时间。You
can't speak even one sentence in Chinese, wasting all of that time
studying.
- 我 去 的 时候 他 不 在 家， 所以 我 白 跑 了 一 趟。He wasn't
home when I got there, so I wasted all of that time going over
there.
- 不好意思， 让 你 白 忙 了 一 个 早上，我们 老板 今天 来 不了 了。Sorry to make you work this whole morning for nothing. Our
boss just said that he couldn't come today.
Structure with 白白
白 can also be replaced with 白白, and they are basically the same,
though 白白 has a stronger emphasis on the "wasted effort." 白白 also
is usually paired with a disyllabic word, like 帮助, 准备, 学习 etc.
Unlike 白, 白白 can be used with a 地.
Examples
- 明天 不 考 中文， 小王 白白 准备 了 一 天。There's
not a Chinese test tomorrow. Little Wang studied the whole day in
vain.
- 他 没有 来，我 在 飞机场 白白 等 了 他 四 个 小时。He
hasn't come. I wasted four hours at the airport waiting for
him.
- 她 白白 学 了 十 年 的 英语， 她 看得懂 英文，但是 连 简单 的 对话 都 不 会
说！She spent ten years fruitlessly studying
English. She can read English, but she can't even speak a simple
sentence.
Sources and further
reading
Category:B2 grammar points