What is the HTML code for new line?

Are you thinking of the “break” tag?

or, more correctly:

This will cause the output after this tag to begin on the “next” text-line. As in…

1. This 
2. text will all 
3. appear as a stream of text, wrapping automatically  
4. only if there isn't enough room  
5. to fit on a single line of output.
   But this text will 
6. be displayed, starting at a new line (even though it  
7. started on the same line of input  
8. as text before the tag. 

Will appear, as follows:

1. This text will all appear as a stream of text, wrapping automatically only if there isn't enough room to fit on a single line of output. 
2. But this text will be displayed, starting at a new line (even though it started on the same line of input as text before the tag. 

The text starting with “But” will start on a new line.

If the content separated by the break-tag are not text, their horizontal flow will still be broken; the elements appearing vertically, above each other.

Note that HTML tags occur in open/close pairs, with an opening tag and and a closing tag, like so:

1. … 

But some tags, like the break-tag, don't have a symantic middle (i.e., no content between them) so they can be abbreviated by combining the opening and closing tags as a single tag: