正确的应该是：I spoke English not only very carefully,but very clearly .（as well可去掉）
not only ...but also...不但...而且...（also可省略）
not only 一般在动词后面,在动词前面时,放句首,句子要倒装,but或but also后面的正常语序.
Not only did I speak English very carefully,but very clearly.