SHOULD I WRITE: Should I write "an Antonio Machado's poem" OR or "an Antonio Machado poem"?
I get confused when it comes to Saxon genitives.
Thanks.
Welcome to the forum.
Please note my corrections above. Don't use capital letters for complete words (except in specific circumstances) - it's the online equivalent of shouting. Make sure you mark out the words you're asking us about in some way - as you can see above, I have enclosed each of your suggestions in quotation marks. One space between words suffices. Don't put a space before a question mark.
You should use (in a complete sentence, of course) "an Antonio Machado poem". The poet's name serves as an adjective there. If you want to use the possessive, you can't use the indefinite article. You would need "one of Antonio Machado's poem
s".
Also note that I have changed your thread title. Titles should be unique, relevant to the thread and contain some/all of the words/phrases/sentences you are asking us about. The subject of your question (Saxon genitive) should appear only in the main body of your post.