According to the descriptive approach, grammar tells us

Grammar is the structural foundation of our ability to express ourselves. The more we are aware of how it works, the more we can monitor the meaning and effectiveness of the way we and others use language. 

Key PointsDescriptive Approach of Grammar:

• Descriptive grammar is also known as Modern grammar. It treats the subject of grammar on inductive lines. It does not focus on how it should be used, however, it focuses on describing the English language as it is used.  
• It refers to describe the patterns of language in actual use. It describes how a language is actually spoken or written and does not prescribe how it ought to be spoken or written. 

Hence, it is clear that According to the descriptive approach, grammar tells us how the language is used.