İstanbullu'yum or İstanbulluyum?

I had decided to share common mistakes that Turkish Learners make. Here we are learning the second one and this is something we native Turkish speakers also do.
When you speak, there is no difference but when you write where you are from, you don't use the punctuation mark.

So, İstanbulluyum is the correct one.

But where does this rule come from?

We add a suffix (-li/lı/lu/lü) to the word İstanbul which changes the meaning of the word stem.
It was İstanbul but İstanbullu is someone from İstanbul. (İzmirli, Ankaralı, Amerikalı, Burdurlu,Gölcüklü)

When you tell a country name, place name or person name,
There exists only one of them. But there lives more than one İstanbullu, so it is not that special anymore?
Well it depends who she/he is and for whom, but Turkish Grammar thinks that they are all equal.

If you want to learn another common mistake, please visit  http://blog.learnturkisheasy.com/2012/10/plural-ending-with-numbers.html