非常感谢! How do you say 'thanks for correcting me' in Mandarin? I want to use it for next time!
在我住的城市，最近经常下雨了!
对, 萬隆是在爪哇岛上。

Thank you for the correction!
In fact, Indonesia has over 17000 islands, but those five are the most prominent ones. There are perhaps thousands of unnamed or uninhabited islands!
I have two questions, if you don't mind.
1. From what I understand, 岛 is a suffix, and 岛屿 is the noun. I can't, for example, say 这个国家有很多岛。Is my understanding correct?
2. Besides paraphrasing, what do Chinese people do when they don't know/forget how to write a character?

I can’t say it is totally wrong, but orally we do say 这个地方由多个岛组成, well to be honest, I use 岛 more often than 岛屿 because I usually just say xx岛 (xx - name of the island) and personally, I think there’s much difference.
Haha for your second question, actually I will forget some characters as well, especially when we write it less often now, because we have computer. But i figured out that for those periods that I read Chinese book (as leisure reading), my Chinese becomes better but for those moments that I am writing english or other languages for most of the time, some Chinese characters just escape from my mind.