Keats imitates the Shakespearean sonnet in his poem "When I Have Fears." His poem perfectly mimics the form of a Shakespearean sonnet, both in rhyme scheme (ABAB CDCD EFEF GG) and form. He uses the standard three quatrains followed by the rhyming couplet, which is how Shakespeare constructed his sonnets. Within the quatrains, Keats uses subordinate clauses, beginning with the word "when," and also "then" in the final couplet.
Moreover, Keats' poem is written in iambic pentameter, which was the standard meter for Shakespearean sonnets. An iamb is composed of an unaccented syllable followed by an accented one, which makes a duh-DA sound. When poets use iambic pentameter, they have five iambs per line of poetry. "When I Have Fears" follows this pattern throughout the entire poem. Here is the final couplet broken down to show the pattern of the unstressed and stressed syllables:
Of the | wide world | I stand | a-lone, | and think
Till love | and fame | to no- | thing-ness | do sink.
Keats clearly carefully studied the Shakespearean sonnet and masterfully replicated its composition in his poem "When I Have Fears."