Rule 1: Every pronoun must have a clear and conspicuous antecedent (word to which it refers). The antecedent must be a single noun, not an entire sentence or idea. Therefore, do not use the pronouns it, which, that, and this to refer to an entire sentence.

Unclear:
Javier wrote to his brother when he was away at college.
My husband wants me to become a teacher, but I’m not interested in it.
At this university, they require students to pass a writing test.
It says in today’s paper that the weather will be warmer.
In the President’s speech, he said Americans need better health care insurance.
In my senior seminar, you have to write three research papers.
Hannah is not doing well in her physics class, which is frustrating.

Clear:
When Javier was away at college, he wrote to his brother.
My husband wants me to become a teacher, but I’m not interested in teaching.
At this university, students are required to pass a writing test.
Today’s paper says that the weather will be warmer.
In his speech, the President said Americans need better health care insurance.
In my senior seminar, students have to write three research papers.
Hannah is not doing well in her physics class, a situation which is frustrating.

Rule 2: Every pronoun must agree with/match its antecedent in number: If the antecedent is singular, the pronoun must be singular too; if the antecedent is plural, the pronoun must be plural, too. And--every pronoun must agree with/match its antecedent in gender: Use masculine, feminine, or neuter pronouns (it or its) to match the masculine, feminine, or neuter antecedents.

The prosecuting attorney presented his case.
Each witness gave his or her testimony.
Judge Judy made her decision.
The jurors gave their opinions.
The court made its decision.
Every journalist wrote his or her story.
The journalists asked each witness for his or her opinion.
The prosecution team has never won any of its cases.