On a single line, for example:

__asm("instruction[;instruction]");
__asm{instruction[;instruction]}

You cannot include comments.

Using multiple adjacent strings, for example:

__asm("ADD x, x, #1\n"
      "MOV y, x\n");

This enables you to use macros to generate inline assembly, for example:

#define ADDLSL(x, y, shift) __asm ("ADD " #x ", " #y ", LSL " #shift)

On multiple lines, for example:

__asm
{
    ...
    instruction
    ...
}

You can use C or C++ comments anywhere in an inline assembly language block.