Admittedly, most writers regardless of experience don’t make this mistake. But enough do that it is worth mentioning here.
Adjectives are words which modify nouns and pronouns. Period. Adverbs, on the other hand, modify verbs as well as adjectives and even other adverbs.
Take a look at the example sentence in the graphic above. Very and great may both appear to be adjectives, but that isn’t so. Great is an adjective, but very is an adverb.
How do you tell the difference? Look at what is being modified. In the case of the example sentence, the modifiers are connected to extent. While you would say great extent, you would not say very extent. That’s because very is an adverb, and adverbs do not modify nouns. Very modifies great which modifies extent.
Of course, this example is rather simple. But so is understanding the difference between adjectives and adverbs. As I said, few writers actually get these two parts of speech confused. But it’s always advisable to check for even the simple things. You never know what mistakes can creep into your writing.