Understanding Common Lisp do macro syntax

(do ((n 0 (1+ n))  ;declares n, initially 0, n+1 each subsequent iteration)
     (cur 0 next)   ;declares cur, initially 0, then old value of next
     (next 1 (+ cur next))) ;declares next, initially 1, then the sum of (the old) cur and next
    ((= 10 n) ;end condition (ends when n = 10)
     cur)    ; return value
  ;empty body
  )

translating into c-like code

for(n=0, cur=0, next=1 ;
    !(n == 10) ;
    n=old_n+1, cur=old_next, next = old_cur + old_next)
{
    //do nothing 
    old_n = n;
    old_cur = cur;
    old_next = next;
}
return cur;

incidentally you should be able to see that this code returns the 10th Fibonacci number

---

Optional EBNF/formal syntax:

The syntax according to the Hyperspec is:

(do ({var | (var [init-form [step-form]])}*) 
    (end-test-form result-form*) 
    declaration* 
    {tag | statement}*)

Understanding this requires knowledge of EBNF and big chunks of the Hyperspec

Your good indentation clearly shows which part is which:

(do ((n 0 (1+ n))
    ^(cur 0 next)
    |(next 1 (+ cur next)))
    |
    +-- first argument of do

    ((= 10 n) cur)))
    ^
    |
    +-- start of second argument of do

Look, they line up nicely, and the inner material is indented:

   ((n 0 (1+ n))
    (cur 0 next)
    (next 1 (+ cur next)))
    ^
    |
    +- inner material of argument: three forms which are
       indented by 1 character and aligned together.

Your do doesn't have a third argument there: there is no body of statements (empty loop).