"Last month I read nine books altogether."
So I tried this: "去月一共我读九本书" and it was marked wrong, but in doing a quick google, I found many examples of English/Mandarin translations where 去月 was translated as last month.
Are you sure it's incorrect?
I've seen 过去一个月 and 过去的月 for "past month" but if I heard "去月" I'd think you're going to the moon.
"上个月我一共看了九本书。" works grammatically, but it would be better if it was 讀了 rather than 看了.
Would it be acceptable to drop the 个 and just have 上月 instead of 上个月？Or is that incorrect?
You really can't do that. The form 上个月 means "one month before this current month". I think, in context, someone would understand what you mean without the 个 but it would be grammatically incorrect.
It denotes action completed. Chinese, along with some other languages, has a notion of aspect rather than tense.
Shouldn't you be able to put 了 after the verb or at the end of the sentence?
To mark the verb as completed you must always put it right after the verb. The end of sentence 了 has a different meaning, for example (among other things) that the sentence describes a change of state.
I wondered the same and looked this up ("Modern Mandarin Chinese Grammar: A Practical Guide" 28.1.3) “了 never occurs between the action verb and the resultative ending," so the action verb (to read) is followed by 了, and the result (the books read) is not. Hope this is right and helps anyone who has a similar question.
Why does the time have to come before the subject in this case? Is 我上个月一共看了九本书 incorrect?