What is an Idiom? 

Idioms are phrases that may not be expressly clear in meaning by simply looking at the comprising words. For example, "How is it going over there?" doesn't really make much sense if we think about what how means. How usually describes a degree or manner of something. But in this context, English speakers will have no problem understanding the writer is intending to ask if you are doing well or not.

Idiom vs. Cliché

Cliché and Idiom may be used interchangeably, but they are not the same. For example: 
"He normally goes to hit the sack before midnight" doesn't mean that anybody is hitting a physical sack. 
The phrase simply means to go to bed and sleep. Clichés are only like idioms in that their words cannot be understood right off the bat by examining each of them.  However, not all idioms are clichés. For example, "in the blink of an eye" is a phrase you are likely familiar with. The meaning of the phase doesn't mean anyone is actually blinking his eye but to do something very quickly or in an instant.  Why an eye? Can you have in the blink of two eyes? You get why they are not understood by looking at the individual words now. 

Other Types of Idiomatic Usage

There are a lot of prepositional idioms in use today. And, to complicate things, many of them have standard prepositions that go with the preceding words. It's very unlikely to choose a preposition and have it go with any word. For example, you cannot say we don't go out in the night, but we don't go out at night.  There's really no hard and fast rule in using prepositions with certain words. Your best approach is to read avidly, and you'll be familiar with matching pairs to use in your writing. 

Commonly Confused Idioms

Arrived in/at?
It's very common for writers to have problems with these phrases since they both have very similar uses. Both have something to do with reaching, coming to or getting to a particular place. However, we use arrive at when we see a destination as a point. If the destination is considered very large, arrive in is what you want to use. So, you can say:
Paul arrived in the country yesterday not Paul arrived at the country yesterday
I arrived at school before classes started not I arrived in school before classes started.
Married to/with?
Many people also fall into the trap of using the word married with the preposition with. This surely doesn't make for good writing. You are married to someone, get married to them, or you are simply married. Thus, we have:
Andy is married to Clara, not Andy is married with Clara.

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