Imaginary Numbers

I have a question--sometimes the answer to the square root of an imaginary number will just be written in positive form with an i, but other times, it will be written like this (ex): 2√−4.  When do I know to write an answer in each form (ex). √-38?  

In other words, when do you know how to write √-38 as 38i or #√# i?

Do you understand what I am asking?  I don´t know if I did a great job of explaining that.