a dilemma

[virginiawang]

I am writing a book now, and I've almost reached the end of it before I discovered that the opening sentence in the first chapter was somewhat ungrammatical. Then I was thinking of making it grammatical by adding a small phrase to specify the time, but as I read the new sentence over, I found that my original feelings were all gone. Now I am debating with myself whether I shoud add this phrase or not.
Do people always use past progressive tense with a phrase that clearly indicates time? Can I say something in this tense with my time blurred or only hinted at by the context?
I was thinking that it will be a joke if I send the book to a publisher with the very first sentence being ungrammatical, but to alter it would make it feel like it was written by someone else. What do you think?

[SleepyWitch]

I don't think you need a time phrase. E.g. "He was sitting at the café." could be the opening for a novel and there's no need to indicate the time at all. The past progressive is used to indicate the background action for a more definite event. E.g. "He was sitting at the café, when a beautiful woman entered."
hope it helps :)

[virginiawang]

Thank you for your advice. I believe you are right. I am not going to add a time phrase to my opening sentence.
May I ask you another question about grammar? I wrote a sentence, but I don't know whther it is grammatical or not after I read it again today. Can I say, "Not until yesterday had I ...when sb did sth"?
Thank you again.

[SleepyWitch]

yes, I believe that's possible. it's called inversion after negative expressions or something. I can look it up for you, if you need 100% certainy. (I'm not a native speaker of English myself).

[virginiawang]

Not until yesterday had I realized how thin I was when I looked into the mirror.
How does it sound? It think it is OK.
I had not realized how thin I was, until I looked into the mirror yesterday.
Does it sound better?
I think they are the same.

I registered with a grammar forum, in which a teacher corrected my sentence in a way that made me feel nothing was changed. What do you think? He said the second one is better. I think the first one was written in a slightly old form of English, but not wrong.

[SleepyWitch]

I think the first one is a bit odd:
Not until yesterday had I realized how thin I was when I looked in the mirror.
By sticking 'when I looked in the mirror' to the end, you make it sounds like 'I was only thin that one time I looked in the mirror, but otherwise I wasn't thin'. this doesn't make much sense. I think you'll either need to move that bit further to the front: Not until yesterday (,) when I looked in the mirror(,) had I realized how thin I was.
or go for a sentence without inversion, like the second sentence you suggested.