Note: If you can't comment on your own post or answers to your own questions because you don't have 50 reputation, please see the "Special Note" at the bottom.
Why does this limit exist?
We realize that new users may have valuable comments, and that we may lose some of those contributions by requiring 50 reputation points to unlock the feature.
However, history and experience have shown that the downsides of allowing everyone to comment are far greater than a few useful comments lost:
There are big problems with spam. Automated filters cannot catch all of it.
Even among the real comments, most would either say "I have the same problem" or "I agree". Such comments do not add any value, and have to be manually removed.
Comments are very painful to moderate. Stack Exchange sites have a process of community moderation (voting, flagging, review queues) that works great for questions and answers, but not so much for comments. Comments cannot be downvoted or closevoted, nor searched (nor do we want that).
Comments are second class citizens on the Stack Exchange network, not designed to hold information for all eternity. They may get cleaned up at any time. Generally, truly important information should be incorporated into an answer anyway (either by posting a new answer, if the information answers the question at least partially, or by editing an existing answer, if the information is a minor complement or clarification of that answer).
Asking good questions and providing good answers will get you 50 rep points in no time. Alternatively, you can suggest edits that improve existing questions and answers. Each of those will gain you 2 reputation points.
But I want to contribute now. What should I do instead?
Can I put my comment in an answer?
Asking and answering, the core actions on the Stack Exchange network of sites, are open to all users regardless of reputation.
However, this is not meant as a workaround to the comments barrier. Comments posted as an answer are subject to removal.
To make requests for clarification, or mere responses to other answers, you need to wait until you have the comment privilege.
How do I write a good answer?
If you are sure you can provide a real answer, then you are of course welcome to go ahead, but make sure it's a full, valid answer to the question. If you saw something wrong with an existing answer, do mention it and its flaws, but make sure you provide an alternative solution that can stand on its own. Do not just copy and paste your original comment into the "answer" field.
But I can't write a good answer without more information!
Answers don't have to be exhaustive or infallible, they just need to try to answer the question. It's perfectly fine to post an answer saying, for example, "I'm not sure what the cause of your problem is, but if it's X, you can solve it by doing Y. If that doesn't help, try Z and let me know what it says." Also, if further information does become available, you can edit your answer later to make it more precise. This is a good habit to get into even if you can comment!
On busier sites, if you cannot even try to answer without asking for clarification, then move on to another question. Many new questions are asked every minute, and there are many unanswered questions still waiting for your help. You might feel bad about abandoning the question, but it is the absolute responsibility of the asker to include all relevant information, presented in a clear manner. Failure to do so at any level decreases their chances to find a good answer to their problem.
Welcome to the community, and good luck!
But I saw a new user posting a comment! How did they do it?
- The user had enough reputation to post a comment at the time, but they lost reputation from downvotes, removed posts, giving bounties, etc.
- The user is currently suspended, causing their reputation to show as 1.
- The user commented on their own question or answer, or any answer to one of the user's own questions. Source
- Here on Meta Stack Exchange, the reputation required to comment is 5, not 50. Source
- The user flagged to close the question as a duplicate, resulting in an automatic comment beginning with "Possible duplicate of".
- The user posted a "trivial answer" consisting only of a link to anywhere in the Stack Exchange network, which the system automatically converted to a comment.
- The user posted the comment as an answer, but a moderator converted it to a comment. Moderators do this only in exceptional cases if they spot an answer that doesn't answer the question, but is still valuable for some reason; such conversions aren't done on request.
- Moderators can choose to place the answer-converted-to-comment on either the question or on any other answer to the question. (source)
Special Note: Commenting on your own posts or answers to your own questions
All users, regardless of reputation, should be able to comment on their own questions and answers, as well as answers to one's own questions. If you can't, it's likely that you have accidentally created a second account. Since your question was posted through (is owned by) a different account, the system doesn't recognize the post as yours and prevents you from commenting without 50 reputation.
To resolve this issue, please contact the team and request that the account you're currently signed into be merged with the account that posted the question. See How can one link / merge / combine / associate two accounts / users? (Anonymous / unregistered / cookie, or OpenID / registered)