1. by half常用来表示程度，用于“too+形容词+by half”结构，表示“过分”。
He is too clever by half. 他聪明得过头了。（=much too clever）
His many admirers describe him as clever: his enemies as too clever by half.
崇拜他的人很多说他聪明；恨他的人则说他聪明过了头。
go too far过分
He's always been quite crude, but this time he's gone too far.
他一向很粗鲁，但这次太过分了。
I can enjoy a joke as well as the next man, but this is going too far.
我和平常人一样喜欢开玩笑，可这太过分了。
Their more recent music is far too commercial.
他们最近的音乐过分商业化了。
提示
by half也可表示“（减少、增加）一半”。
We'll need to reduce the weight by half.
我们得把重量减轻一半。
The workforce has been slashed by half.
职工人数裁减了一半。
Boil for 20 minutes until the liquid has reduced by half.
煮20分钟，直到液体体积减半。
They have reduced the costs by half. 他们把费用减少了一半。
The article is too long and should be cut down by half.
这篇稿子太长，得砍去一半。
Boil the liquid in a small saucepan to reduce it by half.
用小炖锅炖，收一半汤汁。
A tailwind had cut the flying time by half an hour.
由于顺风，飞行时间缩短了半小时。
Cutting food intake by half is an incredibly difficult thing for anyone to do.
将进食量减半对任何人来说都是件非常难的事。
The number of 7 year olds who read poorly has increased by half over the past 5 years.
不识字的7岁儿童的人数在过去5年里增加了一半。
2.in half表示“分成两半”，相当于into two。
I've divided the money in half.
我把钱平均分成了两半。
Cut the cake in half horizontally and spread jam on one half.
把蛋糕横切成两半，把果酱涂在半面上。
Now cut the tomatoes in half.
把西红柿都切成两半。
Cut the banana in half lengthways.
把香蕉竖着切成两半。
Peel the onion and cut it in half lengthwise.
剥掉洋葱皮并把它纵向切成两半。
Cut the tomatoes in half vertically.
把西红柿纵向切成两半。
Please cut the apple in half. 请把苹果切成两半。（=into two）
She sawed the plank in half.
她把木板锯成两截。
Cut a marrow in half and scoop out the seeds.
把一个西葫芦切成两半，挖出种子。
Cut the dough in half and shape each half into a loaf.
将生面团一切为二，再分别揉成长条形。
Divide the pastry in half and roll out each piece。
把油酥面团一分为二，再分别擀成皮。
She gripped the pipe with both hands, trying to snap it in half.
她双手紧握烟斗，想要把它掰成两半。
I once saw a man tear a telephone directory in half.
我曾看见一个人把电话簿撕成了两半。
3.by halves表示“不彻底，半截子，半途而废”，常用于否定句和疑问句中。
A child must never do anything by halves.
小孩做事不可半途而废。
In Italy they rarely do things by halves.
在意大利，人们很少马虎从事。
Better not do it at all than do it by halves. 做事不要虎头蛇尾。
Why should you do it by halves？你为什么不善始善终？