First, go to Tools > AutoCorrect Options and uncheck "Capitalize first
letter of sentences".
But then please train yourself to stop hitting Enter at the end of every
line. Word is NOT a typewriter -- it will automatically wrap words to the
next line when they won't fit within the margins. You should hit Enter only
at the end of each paragraph.
When the AutoCorrect option is turned on, it assumes that each time you hit
Enter you're starting a new paragraph, and each paragraph by definition
starts with a new sentence; that's why it was capitalizing "in
mid-sentence".