Complex sentence: Some also believe that the poem is due to the loss of his son, William in 1819 (to Mary Shelley), his son Charles (to Harriet Shelley) died in 1826, after "Ode to the West Wind" was written and published.
Split into simpler sentences: Some also believe that the poem is due to the loss of his son, William in 1819 (to Mary Shelley). His son Charles (to Harriet Shelley) died in 1826, after "Ode to the West Wind" was written and published.