When writing your SAT essay, avoid pronoun shifts by keeping your pronouns consistent.
A pronoun shift occurs when you start a sentence using one type of pronoun, then shift to another type of pronoun.
This is considered bad grammar because it can confuse the reader as to your intended meaning.
Always be clear who is doing what.
For example, this is wrong:

If one wishes to earn a high score on the SAT, you should study grammar and take practice tests.

The error is a shift in pronoun from one to you.
To fix this, you can change all the pronouns to you:

If you wish to earn a high score on the SAT, you should study grammar and take practice tests.

Or change all the pronouns to one:

If one wishes to earn a high score on the SAT, one should study grammar and take practice tests.

There are other ways to correct the pronoun shift.
For example:

If a person wishes to earn a high score on the SAT, he or she should study grammar and take practice tests.

Here is an example of a grammar mistake due to a pronoun shift from we to you:

When we prepared for the SAT, we realized you could improve your grammar through practice.

Here is a corrected version:

When we prepared for the SAT, we realized we could improve our grammar through practice.

Notice the difference? When double-checking your SAT essays, pay close attention to your use of pronouns in order to avoid the dreaded pronoun shift.