I have had to work this all out for myself. It was ages ago, but it is still something I find myself doing automatically part of the time and stumbling over at other times. Books! Why don't they just carefully explain these tricky things. The waw is sometimes an 'u' and sometimes a 'w'. Just because of the unwritten vowel immediately before it.
sura and suwar, صور ,صورة.
Also, the emphasis is on a particular vowel in each word. I have picked this up instinctively, but it would have been really helpful to find it maybe once or twice in the book. Ok, I have got it all by myself.
sUra and suwAr
How do I know? I just do. Probably from hearing it, but also from hearing other singular/plural combinations.
I need a drop-in: "Learning Arabic? So you will be needing coffee and a rant every week? I knew it. Well, here it is."