Fixing text typed in capitals or in title case
Two situations account for most uses. The first is text typed with caps lock on, or copied from a source that shouts: a whole paragraph comes back as ordinary sentences. The second is a list of headings in Title Case that a style now wants in sentence case: 'How To Write A Good Headline For NASA' becomes 'How to write a good headline for NASA'.
In the second case words with capitals inside, such as NASA, survive because Keep words that already have capitals inside is on. Plain names such as London are written as London in the input and as london in the output, because to the converter London is just a word that starts with a capital, like How or Write. Put those capitals back by hand. In text that was entirely in capitals there is nothing to keep, so every word is lowered first.
Where sentence case is the house style
Several style guides for software documentation, including Microsoft's writing style guide and Google's developer documentation style guide, ask for sentence case in headings and titles. It is also the natural choice for emails, subject lines, chat messages and anything that should read like speech. Title case tends to appear in book titles, article headlines and the names of products and sections that a publication has chosen to capitalize.
Whichever you choose, use it everywhere in one document. If you need the other style, the title case converter does the reverse job.
How sentence ends are found
A sentence ends at a full stop, question mark, exclamation mark or ellipsis, and at the matching marks used in Chinese and Japanese. The next letter after a space or line break gets the capital. A closing quote or bracket after the mark stays with the sentence: in 'He said "stop." Then he left', the T of Then is capitalized, and a single quote works the same way. A quote mark in front of a word does not hide it: "'hello' she said" becomes "'Hello' she said".
Numbers, decimals and web addresses do not end sentences, because no space follows their dots. A list such as '1. buy milk' becomes '1. Buy milk'. A few abbreviations, namely Dr., Mr., Mrs., Ms., Prof., Sr., Jr., St., vs., e.g. and i.e., and dotted initialisms such as U.S., are recognized and do not start a new sentence. Any other abbreviation followed by a full stop and a space is treated as the end of a sentence, so check text that is full of them.
Languages where sentence case does not fit
Sentence case lowers every word that is not at the start of a sentence. That is right for English, but German writes all nouns with a capital, so converting German text this way damages it. Languages without capitals, such as Chinese, Japanese and Arabic, are unchanged. The pronoun rule is for English only; switch it off for other languages.