Substitution involves replacing one term for another in an equation or system of equations. To solve a system using substitution, one equation is solved for one variable and substituted into the other equation. This creates a single variable equation that can then be solved. Parallel lines yield a false statement when substituted into each other, indicating they have no solution, while the same line yields a true statement and infinite solutions. When choosing a variable to substitute, select the one without a coefficient to more easily substitute terms.