This one will be all about idioms. You may have heard about them. If not, we'll give you a primer. No worries.
So what are idioms? If you want the dictionary definition, here it is:
A speech form or an expression of a given language that is peculiar to itself grammatically or cannot be understood from the individual meanings of its elements.
An example of this is, "That's the way the cookie crumbles." Or, "My bad." Anyone who is a native English speaker in the U.S. doesn't even bat an eye at this saying. A non-native speaker who is fluent in the language may scratch his/her head at it. That's common with idioms: They're confusing to everyone who doesn't have a solid grasp of the languge. Hell, some idioms are regional, so they can be confusing to some native speakers and not others. (A recent example of this was my using, "Don't look a gift horse in the mouth," with my husband, to which he looked perplexed.
It's important to note that idioms aren't necessarily grammatically bad sentences. Many of them show complete mastery of English grammar. It's just that the words used to communicate them can't always be taken literally.
So how does this relate with writing? Well, idioms are a perfectly reasonable way to express yourself. In fact, there are times when an idiom does the trick much more efficently than standard language. But in general, you want to avoid them when writing formally. Unless you are talking about the idiom in your writing, it shouldn't show up in a business report ready to be turned in to your manager. It definitely should stay out of academic papers unless, again, you're writing a linguistics paper on idioms.
Some would argue that idioms are a lazy way to speak. I disagree with that assessment. I think they can sometimes be a crutch, mainly because people use them so much. Most people don't set out to avoid speaking literally. But you can resolve that by learning alternative terms. A good way to practice this is by speaking literally, that is, using the words you use mean what they are supposed to mean.
So "That's the way the cookie crumbles!" can be tranformed to "That happens sometimes!" Here are some more examples:
"He's got his ducks in a row" = "He's an organized person."
"She bought the farm" = "She died."
"They made a mountain out of a mole hill" = "They overreacted."
"Don't look a gift horse in the mount" = "Make sure to take advantage of that."
"It is what it is." = "I can't do anything about that."
"Those people have sticky fingers." = "Those people are thieves."
"Your goose is cooked." = "It's over for you."
The list goes on and on, of course. The name of the game is to remember and be mindful of your language. Sometimes, they do the trick (an idiom!); just use them sparingly, and definitely limit raining them down on your professional or scholastic documents.