οὐκ ἦν ἐκεῖνος τὸ φῶς, ἀλλ᾽ ἵνα μαρτυρήσῃ περὶ τοῦ φωτός.

1. οὐκ
   not
2. ἦν
   was
3. ἐκεῖνος
   that one
   that
4. τὸ
   the
5. φῶς
   light
6.  
   ,
    
7. ἀλλ᾽
   but
8. ἵνα
   so that
9. μαρτυρήσῃ
   he might bear witness
   he might testify, might testify
10. περὶ
    about
    concerning
11. τοῦ
    of the
    the
12. φωτός
    light
13.  
    .
     

He was not the light, but came to testify concerning the light.