What are Prepositions? 

Prepositions are words used to show relationships between other words in order to convey a precise meaning in a sentence. Thus, they essentially let you know when or where something is happening. However, many prepositions can assume different definitions depending on the context. It is also not a grammatical flaw to end a sentence with a preposition.  
Prepositions, for example, may inform where a particular noun is, relative to another noun. Consider the statement. 
Your mobile phone is on the desk. 
Here, you might have been looking for your phone only to find out that your phone (a noun) is on another noun (the desk), in which case the preposition "on" tells you the relative position of your phone to the desk. 

Types of Prepositions
There are different types of prepositions since they inform the location, time, direction or spatial and abstract relationships.
Direction: The team's best player lives beside me
Time:  He came after the group's training session
Location: I saw his car at the park
Space: The cat was inadvertently squeezed under the pillow

Preposition Examples
There's particularly no sure-fire guideline when looking for a preposition that goes well with a set of words. However, by consistently writing and reading works of experienced writers, you'll undoubtedly take giant strides in improving the quality of writings you churn out while using prepositions. 
Google Ngrams is a great tool to learn which prepositions go with certain words. However, the only caveat is that it does not clarify any difference in meaning between different prepositional phrases.

Ending a Sentence With a Preposition
It was traditionally believed to be wrong to use prepositions when completing a sentence. But we know it definitely couldn't be further from the truth. Those who go with this claim often have to deal with slightly unnatural and awkward sentences. Let's look at the example below;
He found no close friend to stay beside (More grammatically natural and correct).
He found no close friend beside whom to stay (Grammatically correct but awkward).
But sure enough, there are times when moving a preposition from the end of a sentence is preferred. For example, if you are writing for a formal occasion moving the preposition from the end is preferred.

Unnecessary Prepositions
There are quite some redundant ways to use a preposition. A common example is the use of "at" when ending a question.
Where's your uncle at?
While this is very common in some dialects, you'll want to do away with such redundancy in writing as it is considered an error. A quick fix can be made by deleting the unnecessary prepositon. So, where's your uncle? is just fine.
There are many other cases where you might find it slightly difficult knowing which prepositions to use. But thankfully, you can avoid them altogether for a more concise and free-flowing sentence. 
Getting rid prepositions is something you want if they are not vitally important. 
Consider the following sentence. 
In most cases, enrolling in an academic course alongside other challenging extracurricular activities is a recipe for a number of concerns for many people.
While you may not have problems with the sentence, making a more concise statement by deleting the unnecessary prepositions is surely a good idea to keep your ideas simple. So; 
"Many people find combining academic and extracurricular activities an upheaval task" likely sounds better. 

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