-"İngilizce o kadar da zor değil" -English is not as hard as you (guess/think). -English is not 'that= o kadar da', (imaginary high level' =as much as.) -actually you are asking a hard question here; because I could use this sentence:'ingilizce o kadar zor değil' without 'da'. it is a 'Connector' to make sentence's meaning more stronger, but not necessary

It means, english is hard, but not hard you think. Here, "o kadar"(that count) ls a level in someones mind, it is an indicator, in this sentence it indicatas the hardness level of english. But, in reality, if you compare them, okay english is hard, but not too hard as you think.

'da' is conjuction in Turkish language. It has not have specific meaning. At this sentence, 'da' makes to strengthen subject's opinion