That doesn't happen. A common denominator is the result of combining two or more denominators.
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When the numerator is less than the denominator then it is a common fraction but if the numerator is greater than the denominator then it is an improper fraction.
Multiply the numerator of the fraction by the same amount that you multiply the denominator of that fraction.
When a the GCF of the numerator and denominator of a fraction is 1, the fraction is in its simplest form.
First, find a common denominator for the two (or more) fractions. Then, for each fraction, multiply numerator and denominator by the same number (different numbers for different fractions, though), to convert to the common denominator.
That is why it is called an improper fraction because a proper or a common fraction always has its numerator less than its denominator.