What is the hardest language skill to master when learning English: reading, writing, listening, speaking or grammar? And do you think language ...

Pronunciation, without doubt. 

I know spanish and portuguese speaking students have a rough time with the adjectives before noun part, use of prepositions in english, and many other broken rules only a native would know.

for example, carro azul is blue car.

The hardest part of english language is GRAMMAR. I started to learn and study english when I was 7 yrs old, continued with swedish, german and latin. Not to forget my mother language finnish. Rigth now I'm totally fluent with english and I can't let you go without saying how much studying latin helped me. Pronounciation is hard too, but I started so early that english is like my other mother language.

Jaana