This is simple, if this documentation tells you to write ABCD you should probably not write ADCD. It is very simple to see they are not the same. Now, sometimes you will find something like Hello, [mammal]! ... Bye!, so you may replace the [mammal] part by the name of any animal species which females feed their babies with their own breast milk, and the three dots by anything. So Hello, cat! My dog eats grass. Bye! is compliant with the Hello, [mammal]! ... Bye! notation. I replaced [mammal] by cat and the three dots by My dog eats grass. and we can all agree that this is true.
I will really thank you if you are proactive and by simple logic exercises you find the way of writting a valid HTML document. It is not that hard, just think a little. HTML is the easiest language we will learn so treat it as the easy thing it is and do not be scared.