When I have fears that I may cease to be
Before my pen has glean'd my teeming brain,
glean'd=gathered (poems) from
teeming=abundantly filled
Before high piled books, in charact'ry,
charactery=written symbols (letters and numbers)
Hold like rich garners the full-ripen'd grain;
garners=grain bins
full-ripened grain=the poet's mature poems
When I behold, upon the night's starr'd face
Huge cloudy symbols of a high romance
cloudy symbols=clouds that are also symbols
high=sublime
romance=fanciful thoughts of the Romantic poets
And feel that I may never live to trace
Their shadows, with the magic hand of chance;
trace their shadows=put their shapes into poetry
magic hand of chance=poet's bestowed gift of writing poetry
And when I feel, fair creature of an hour!
That I shall never look upon thee more,
Never have relish in the faery power
faery=magical, playful
Of unreflecting love, then on the shore
unreflecting=unstudied
Of the wide world I stand alone, and think,
Till Love and Fame to nothingness do sink.
to nothingness do sink=drift into obscurity/lose their importance