If the research prediction is that the treatment will decrease scores,then the critical region for a directional test will be in the left-hand tail.

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If the research prediction is that the treatment will decrease scores, then the alternative hypothesis will be one-tailed (in the left-hand tail) and the critical region for a directional test will also be in the left-hand tail. This is because we are only interested in the possibility of scores decreasing, and we want to reject the null hypothesis if we have strong evidence that the treatment actually caused a decrease.

If the Research Prediction Is That the Treatment Will Decrease