It was obvious that he wrote a lot. His experiences in WWI as an ambulance driver provided him with much material. His style had been crafted into a specific form, light on punctuation and heavy on conjunctions. This tendency is called polysyndeton. It is found in the King James Bible.
An example of Hemingway’s usage:
“The dog kept close behind him and when David stopped the dog pressed his muzzle into the back of his knee.”-An African Story
The use of the many conjunctions changes the rhythm and in this case heightens tension. Instead of pauses commanded by commas we are forced to keep reading. Hemingway reserves his punctuation for emphasis, such as in this sentence:
“But I never knew anyone else that could shoot better at ten than this boy could; not just show-off shooting, but shooting in competition with grown men and professionals.”
–I Guess Everything Reminds You of Something
His semicolon is usually reserved to discuss matters of serious importance, such as violence or life and death. Commas emphasize a clause.These are good tools to remember when you are writing.