As @Peter mentioned, there is a lot of material relating to "Writing for the Web" on the Nielsen Norman Group site. The overall message is to keep it succinct and to the point, even more so when designing for mobile devices.
Along the same vein of being succinct, you may be interested in an analysis of the BBC News headlines mentioned in a 2009 edition of Jacob Nielsen's Alertbox. The headline writers also have to deal with grammar usage (among many different constraints). I post a short extract, but highly recommend that you read the original article as it contains links to further reading material.
It's hard enough to write for the Web and meet the guidelines for
concise, scannable, and objective content. It's even harder to write
Web headlines, which must be:
- short (because people don't read much online);
- rich in information scent, clearly summarizing the target article;
- front-loaded with the most important keywords (because users often scan only the beginning of list items);
- understandable out of context (because headlines often appear without articles, as in search engine results); and
- predictable, so users know whether they'll like the full article before they click (because people don't return to sites that promise more than they deliver).
The take-away for me, as it relates to your question, is that you should determine if the user needs additional words to understand or predict what you are trying to communicate. I think arguments about using "one word" vs "two words" for a particular button could be resolved by gathering data from A/B tests. When it comes to single sentences, refer to the aforementioned articles for guidance, and try to be succinct.