Word 2013 and Word 2010 provide a feature that automatically capitalizes the first letter of every sentence and by default this feature is automatically enabled. This feature can be enabled or disabled using the Autocorrect Options available as part of the Proofing section. To disable the capitalize the first letter of every sentence feature
Step 1: Click the File menu
Step 2: Select options from the menu list
Step 3: Navigate to Proofing section in the Word Options window.
Step 4: Click the Autocorrect Options button, this would launch the following Autocorrect Options window.
Step 5: Under the AutoCorrect tab, un mark the checkbox with label as Capitalize first letter of sentences and click Ok button to confirm and save the changes.