The Grammar Police
Google Docs is built with aggressive auto-correction features to help writers avoid basic grammatical errors. The most common of these is automatic capitalization. If you type a word, press the period key, hit the spacebar, and begin typing a new word, Docs will instantly and automatically capitalize the first letter of that new word, assuming it is the start of a new sentence. While helpful for writing standard prose, this is incredibly annoying if you are writing lines of code (where case sensitivity is critical), entering data into a specific format, or writing stylized poetry where you intentionally want lowercase letters following punctuation.
Disabling Auto-Capitalization
To maintain absolute control over your text case and stop Google Docs from automatically capitalizing the first letters of words after a period, you must disable the specific autocorrect preference.
Open your Google Doc. Navigate to the top menu bar and click on Tools. From the drop-down list, select Preferences. A new dialogue box will appear with two tabs at the top. Ensure you are on the General tab. Look at the very top of the list of checkboxes. The first option is explicitly labelled Automatically capitalize words. Click the checkbox next to it to remove the checkmark and disable the feature. Click the blue OK button. From this point forward, Google Docs will respect your exact keystrokes. If you type a lowercase letter after a period, it will remain lowercase permanently.