@Rathony,
Rathony wrote:
You google "I have doubt on", you hit 29,400 results while you google "I have doubt about", you hit 54,200. It is more than a half.
Which only proves a lot of people don't know how to use English properly. Google produces a lot of search results written by very stupid people. And googling
I have doubt about instead of
I have doubts about is only going to add to your confusion.
I'm with Setanta on the first phrase.
Doubts about sounds more natural, and is definitely what an English person would say.
As far as
On study goes I would change it to
While studying. Setanta is right, the meaning is not clear so all I can do is come up with a best guess.
The third phrase
write with it is OK, but
write in it sounds more natural, in the birthplace of Shakespeare at least.
You are clearly not a native English speaker, that's glaringly obvious to anyone who has English as their first language.