Users of the Probability Approach choose the right one from four basic types of conditional sentences--factual, predictive, hypothetical and counterfactual conditionals, by judging how likely (i.e. the probability) the event in the result-clause will take place when the condition in the if-clause is met. Thirty-three students from the experimental class received a 30-minute lecture on the Probability Approach between taking two conditional quizzes while 32 students from the control class did not. Averagely speaking, students who learned the Probability Approach were found to have scored 6 points higher in the second quiz than in the first quiz, compared with those who didn't learn this approach. This finding proved that the Probability Approach was effective in helping students acquire the four basic types of conditional sentences.