Grammer!

For me it's the grammar and tenses.

the hardest thing about it is to speak it as native. it never happens so don't put too much effort in it.. hardest while learning: grammar (if it's a european language) and the characters if it's chinese ( <3 ) easiest : pronunciation ( if it's european) nothing if it's chinese ^^

May be the environment! If people around you all speak,write,read a kind of language,i think it's easy for you to learn it : Just know how hard it is to learn english in China~! haha!

learning characters in Chinese. word's gender and endings in Russian