Even if your sentences are grammatically correct, they could still need some work. Repeating identical sentence structures will make your paragraphs sound stiff and unsophisicated. The following links can help you understand how to vary your sentence structure.
Why should I have a variety of sentence types in my paragraphs?
Learn to identify sentence patterns
How to combine sentences for efficiency
Errors like faulty parallelism and misplaced modifiers will make your sentences difficult to read.
Parallel sentence structure: overview and rules
