Answer:
One bad test shouldn't affect your grade.
Explanation:
There needs to be an apostrophe in the word "shouldn't" in order to make it a valid contraction. This is a contraction of the phrase "should not".
The second error is that the word should be "affect" rather than effect. Affect is usually a verb, and it means to impact or change. Effect is usually a noun, an effect is the result of a change.
