What is the show preview button?
Show preview and Show changes buttons
Preview warning
The Show preview button displays what the article will look like without actually saving it. It is strongly recommended that you use this prior to saving since this allows you to check for errors and unexpected results. The Show changes button displays differences between your edits and the previously saved version.
Both allow users to save multiple changes, especially small fixes, in a single edit instead of editing and saving multiple times.
How do I use Show preview?
Instead of clicking "Save page", click "Show preview". You will see a page with an edit box and a copy of the page as it will look upon saving. This has a red "preview warning" at the top to help you remember that you have not saved.
You can preview as many times as you like, until you are satisfied with the look of the page; you must then click "Save page" to submit your edits to the database.
You will also see a small preview of your edit summary just after the summary box, which can be helpful if you have included links or other markup in your summary.
How do I use Show changes?
The "diff" displayed by Show changes
The "Show changes" button displays and highlights the differences between your version and the last saved version. Some people find this easier to examine for errors and completeness than a simple preview. Like preview, you must click "Save page" after showing changes to save your edits permanently.
See Help:Diff for more information on reading the display of differences.
Why should I use Show preview or Show changes?
Saving the same article a large number of times in quick succession makes it harder for people to check what changed between edits, and fills up the page history with unnecessary entries.
Saving less often is also a way of avoiding edit conflicts, as people will not see your first save on recent changes, and therefore they are less likely to try editing it at the same time as you.
What will and won't show in a preview?
Piped links, signatures, and templates, (including substituted templates) will be properly displayed in the preview, but are not yet expanded in the edit box. If you save the page and then edit it again, you will see the expanded versions.
For more information, see Help:Automatic conversion of wikitext
How can I customize my page previews?
If you are logged in, you have two options on the "Editing" tab in your preferences.
"Show preview on first edit" will always display a preview on your first completed edit, before allowing you to save, even if you click "Save page".
"Show preview before edit box" will display the preview above the edit box instead of below it, so that you can scroll down while looking at the preview and then continue editing without needing to scroll back up.
Using preview to check login status
Using show preview also prevents your login timing out, which would mean your edits would be attributed to your IP address instead of your user name. Clicking preview regularly will restart the timer, so you remain logged in. Also, if you are no longer logged in, showing a preview will make you aware of this so you can log in before saving your work.
If you do not see your user name in the upper-right corner when you press preview, you have been logged out. You can open Special:Userlogin in a separate window (right-click "Log in" and hit "Open link in new window" or similar). Log in, then go back to your editing and hit "Show preview" again to make sure all is well.
Note that this will clear the minor edit and watch checkboxes, so make sure they're set appropriately again before you finally save the page.