English Grammar Rules & Mistakes: Learn All of the Essentials: Writing, Speaking, Literature and Punctuation Rules Complete with 10 Key Skills and Over 200 Common Error Examples

Both Jessica and Juan crossed the road.
This example clearly demonstrates that two people crossed the road with the
use of the conjunction both and the conjunction and in a pair. It is a useful
device to use correlative conjunctions as it makes certain that no other
meaning could be understood from this sentence. When we write anything,
having another person understand exactly what we mean is the ultimate goal. 
It is clear that we can’t do without conjunctions when we speak. They are the
literal glue of the English language and allow us to be really clear in what we
are saying.
Making relationships: understanding prepositions
When we have a group of words that might be a combination of verbs, nouns,
adjectives, and adverbs, they are trying to form some sort of meaning
together but to do this they need a little help. What these types of words lack
is the ability to show how they relate to other words or phrases properly. This
is where prepositions come in. A preposition is usually a small word that is
inserted into the sentence to demonstrate a relationship between a noun and
some other word in the sentence.
Jessica crossed the road with Juan.
In this example, the word with is the preposition as it demonstrates the
relationship between Jessica and Juan in that they crossed the road together. 
The horse ran the race despite having an injury.

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This is another example to show that there is a range of different prepositions
and that the placing of them in the sentence is really important. In this
sentence, the meaning is demonstrating that the horse did something despite
something else. The first thing, running the race, has a direct relationship to
the second thing, having an injury, because of the preposition despite.  
You could write this example as two different sentences but the relationship
between the two phrases would not be made clear.
The horse ran the race. The horse has an injury.
As you can see, the impact and meaning have been lost. This shows the
importance of prepositions in establishing an understanding of what is really
trying to be said.
Another important thing to understand about prepositions is that they don’t
always appear where you expect them to be, yet this does not affect the
sentence’s meaning.
The dog is behind the chair.
Behind the chair is the dog.
These two sentences mean exactly the same thing and are both grammatically
correct. However, I would suggest that to make certain your intended
meaning is conveyed correctly, that you try to insert prepositions in the
middle of a sentence. Think about what thing you want to relate to another
thing and write two simple sentences about them. Now take a preposition and
try to insert it in the middle to make a complex sentence.
I studied math. I went to high school.
I studied math throughout high school.
These two sentences have been combined to show that you not only studied
maths but that you studied it throughout your time at high school.
There is quite a large list of prepositions and sometimes people forget which
is one and which isn’t one. The number one rule to remember, if you can’t
connect two simple sentences together with the word then it won’t be a

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preposition. All prepositions show a relationship between the words and
phrases.