Being shown a few examples is not the same as understanding them. If the first time you see an example of the rule is during the explanation of the rule, you have to deal with too much new information at once. A good explanation should refer mostly to information the learner is already familiar with.
The first time you see an example of the rule, there's no need to explain the rule yet. You can just memorize the example. The same is probably true of the second and third example. Explaining the rule only makes sense once remembering the explanation becomes easier than remembering the set of examples you need to know.
Good textbooks already do exactly this. Show a few examples of particular sentence structure, using vocabulary and syntax at the level of the learner, and then briefly draw attention to a general rule at the end.
The first time you see an example of the rule, there's no need to explain the rule yet. You can just memorize the example. The same is probably true of the second and third example. Explaining the rule only makes sense once remembering the explanation becomes easier than remembering the set of examples you need to know.