Students often use an if block when they mean to use a forever-if block. Help students recognize this by saying “if you put a play-note block in the if, how many times would you hear it?” This style of prompt can be a great hint for students and you can use play-note and say blocks to help visualize program execution. Some programming languages use “print-statements” as a similar strategy.
Example: "Add different play note blocks to see how your code works."
See this and related tips as a Tip Sheet: http://csteachingtips.org/tips-for-teaching-scratch