The foundation of every single copy written in the world.

Keep it simple.

The simpler the copy is, the more your prospects read, the more your prospects read the more likely you are to persuading them into action.

Imagine you're writing for a very young audience. If you think they'd understand, then readability is spot on.

Write to the chimpanzee brain. Simply. Directly
-Eugene Schwartz

If the main objective for copy is to persuade and make prospects act, we need to make text have the best odds of persuading and this is by writing it simply.

Readability improves because sentences are thoughts, and people are lazy. Simple sentences mean simple thoughts. Simple thoughts increase readability, and readability increases likelihood of succesful persuasion.

You can measure "readability" with a Flesch-Kinkaid formula.

https://goodcalculators.com/flesch-kincaid-calculator/

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Reference:

Cashvertising pg. 80