For avoiding repetition when writing, subject pronouns are commonly used to substitute (I, you, he, she, it, we, they) when you write them BEFORE the verb. If not, object pronouns (me, you, him, her, it, us, them) is the option when the action goes directly into de subject.
Also the use of indefinite articles (a/an) change when you know what you are referring to, to definite articles (the).
Examples with pronouns:
There was a big dog in the street. It was playing with a cat.
Example with articles:
There was a big dog in the street. The dog was playing with a cat.