Hemingway got away with short sentences and sometimes they are the best option. When are they not the best option? I find if you have opened a scene up and you were descriptive to begin with you can get away with shorter sentences that follow. Some descriptions though, drag on and on! Don’t be a dragger! Say what you need to say and move on. Kind of like shopping in Walmart- make a list then get in and out! Otherwise, when you are at the checkout and she gives you a total of seventy-plus dollars you think to yourself, ‘What did I buy!’ This is the same with sentence structure– Drop your description or the point you are trying to make and don’t dilly~dally!