Question

"if i had not started studied english i would have much difficult nowadays"

a frase acima está gramaticamente correta com o uso do third conditional?

Answer 1

A frase correta seria "if i had not started to study english...."

Answer 2

Não. A estrutura da terceira condicional  é: if + past perfect , would have + partícipio passado.

Para que a frase fique na terceira condicional, teria que ser assim: If I hadn't started studying English, I would've had more difficulty.

Honestamente, a frase desta maneira soa estranho; por isso eu preferiria usá-la com a mixed conditional (hadn't I started studying English, I'd have much difficulty nowadays).