Great writers are made of great sentences. But if you want to improve your sentences, it won't help to fuss over surface-level features, like commas and semicolons or sentence length and vocabulary. In fact, that'll just hold you back from what really counts.
Grammar and vocabulary are the raw ingredients of a sentence. You can no more write a great sentence from formal structures than cook a great meal from an ingredients list.
So if grammar isn't the key to great sentences, what is?
Sentence Patterns for Writers
Ten Great Sentence Structures, and How to Practice Them
William Fitzpatrick