i'm wondering This is a short form for 'I am wondering' You would not say 'I wondering' as that is not a normal form of expression. The short form is indicated by the ' charachter. The short form is known as an abbreviation.

I'm wondering means, I am having doubt in the moment I'm wondering if he's coming. I wonder how it does that Not I wondering but "I wonder" is an expression, an exclamation to call