Were’ needs to be replaced with ‘was’ to make the sentence grammatically correct. This is because,‘If the subjects are joined by as well as, with, along with, together with, and not, in addition to, but, besides, except, rather than, accompanied by, like, unlike, no less than, nothing but, led by, headed by, guided by, controlled by, governed by,etc..Then the verb will agree with the first subject’ Thus, the correct sentence would be, ‘Coleridge as well as Wordsworth was of the opinion that the opposite of poetry os not prose but science’.
