Keats used the sonnet form to compose many of his poems. He commonly used the Shakespearean/English sonnet form, as well as the Italian Petrarchan sonnet.
A sonnet is typically composed in iambic pentameter. Iambic pentameter is a type of meter in which the ten syllables employed in each line are divided into five pairs ("iambs" or "iambic feet"). The second beat of each iamb is stressed.
Shakespearean sonnets are fourteen lines long. The first twelve lines are divided into three stanzas of four lines each: three quatrains. The rhyme scheme of these initial stanzas is: ABAB CDCD EFEF. The sonnet closes with a final rhyming couplet (GG). The Petrarchan sonnet is divided into a rhyming octave, ABBAABBA, and a rhyming sestet, CDCDCD.
Write your own sonnet in the tradition of Keats, using one or more of his thematic elements: romantic love, the dominance of quickly-changing moods, the interplay of mortality and timelessness, and the search for beauty.