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

The replacing block: understanding pronouns
We just learned about how much we love naming things and what a noun is.
So, what if I were to tell you that actually sometimes we don’t want to name
things and instead replace the actual name of something with a generic term
that isn’t at all descriptive or creative? Well, the answer is yes. Sometimes,
we get a bit lazy. Scratch that, we often get pretty lazy and want to shorten
our language so that we can quickly deliver the message of what we want to
say.
Jessica and Juan walked across the road.
In this sentence, we can clearly see two proper nouns, Jessica and Juan, just
as we have learned in the noun section of this book. Now, if we want to say
this sentence more quickly, what would we write?
They walked across the road.
Very easily, we have replaced Jessica and Juan with they. By doing this, we
have replaced the nouns with a pronoun. 
Pronouns act just like nouns, they are a naming word. However, pronouns are
a shortened version of a noun and replace the name. You will see pronouns in
many sentences and we particularly use them when speaking. Pronouns
include I, me, she, he, it, they, us, them, that, this and these. You can always
spot a pronoun because it is not a specific name but if you wanted to you
could replace it with a specific name.

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She loves running.
Jessica loves running.
As you can see, she is easily replaced by Jessica. This shows you that it is a
pronoun. The same goes if you are trying to work out whether you can
replace a noun with a pronoun.
The dog is a Chihuahua.
It is a Chihuahua.
Returning to the example from before, the noun dog is easily replaced with
the pronoun it. Yet, the proper noun Chihuahua can’t be replaced with a
pronoun as it would not make any sense.