When I have fears that I may cease to be
Before my pen has glean'd my teeming brain,
Before high piled books, in charact'ry,
Hold like rich garners the full-ripen'd grain;
The first  quatrain  begins abruptly; Keats never wastes words. The reader is immediately hit by the powerful idea that the young poet is facing his own death; a deeply personal tussle with fear.  As explained in the summary, he was at the time nursing his brother, Tom, who was dying of tuberculosis. It was known at the time that this was an infectious disease that was usually fatal. 
 The first two words “When I” are repeated at the beginning of the beginning of the next two  quatrains , forming a refrain or  anaphora . The effect is to build suspense so that the reader wants to know … and then? 
 For all its honest acceptance of the inevitable, Keats uses a  euphemism  — ‘cease to be’. His stoical acceptance didn’t preclude ‘fears’.