I know that Scrivener really, really wants to leave the first line of a page unindented. But this particular page starts with a poem, a left-justified poem so I can’t use “Verse” style which centers the text. When I compile to PDF, each line of the poem except the first is indented! I’ve tried zillions of versions of changing the indents, tabs, compile settings … I just can’t figure it out. Please tell me how to indent that first line of the first paragraph on a page. Thank you.
Did you start with the Poem project template? Most of the templates definitely are more oriented toward traditional book formatting, and that goes for the built-in formats as well. But the “Poem” template, found in the Miscellaneous category, has a special compile format that should work better for most poetry—mainly in that it has very few opinions about what your text should look like! 
The only thing it does do is drop to Courier for submission purposes, but that’s something you can easily change at the very top of the preview column in the middle of the compile overview screen.
Well, if you did not, that’s no big deal. You could create a quick temporary project using that template, open compile, and double-click on the “Poem” compile format to edit it, setting its “Save to” property to “My Formats”, which will make it a globally available. You can close and trash the project, and make use of this format from your own project.
There is a setting in compile to remove first-line indent after a blank line. I bet you have that option checked and that there is a blank line before the poem. You coukd uncheck that (though it may have have other effects if you do).
Alternatively, you can probably put a “non-breaking” space on the empty line that occurs before the poem to forestall the function mentioned above. More alternatively, maybe you can just not have an empty line before the poem.
The poem is on one page of a book of regular prose. I did, however, finally manage to trick it by inserting a line of just spaces that Scrivener thinks is the first line of the paragraph.
Oh okay, so a verse in the middle of narrative. In that case, for future reference I would consider using a style, particularly if the lines within the verse are all uniform rather than having creative indents. Styles are your go-to for enforcing stricter formatting that should pass through most of the compiler’s settings for cleaning up the text and making it look uniform.