To cut back與To cut in的差別

To cut back指減少，多半只生產方面(to reduce, generally refer to production)
例如：
Due to a trade  downturn, the factory has to cut back(on) production.
由於經貿欠佳，工廠必須減少生產。(on可以省去)
If she doesn't cut back her expenses, she may face financial difficulty.
如果她不減少開支，就會面臨財政困難。
而to cut down是指消費(consumption)方面的減少：
In order to kept Maggie slender, we always cuts down her meat intake.
為了幫馬基(Maggie)保持身材苗條，我們幫她減少肉食。
By cutting down on luxuries, I may save more money for a rainy day.
減少購買奢侈品，我能省更多錢，以備不時之需。
但to cut in是指打斷或開車時超前(to interrupt or sharply turn the car in front of other's)
例如：
Maggie, it is always rude to cut in other people's conversation without permission.
馬基(Maggie)，未經允許打斷別人說話是一直都是非常不禮貌的行為。
Eric, it is indeed really dangerous to cut in while driving on the highway.
艾利克(Eric)，在高速公路上急轉超前的確非常危險。