One-word answers are definitely not acceptable. (In fact, the software doesn't even allow them).
A question that is formulated in a way that seems to call for a one-word answer may need some improvement. For example, if you are unsure what kind of answer the questioner really wants, you might post a comment asking for more detail about the problem or vote to close the question as unclear or not showing research.
But if you do think the question is clear, within the scope of the site, and ready to be answered, here is how you should write an answer post. In addition to saying "yes" or "no" or "sometimes," you should explain why the answer is yes or no. If the explanation is controversial or not obvious to native speakers, you should cite reliable sources about English language and usage to support your answer. (If the answer is obvious to native speakers, it's possible the question actually belongs on the English Language Learners site.)
Other things that might be in a good answer:
- examples from real-life usage to illustrate the point
- an explanation of how the specific question connects to a broader concept
- links to resources that can be used for more in-depth study of the topic
