In your sentence, the correct words you should use is 'not as hard' or 'not as difficult.' You are comparing two things. -English is not as hard as Chinese. -English is not as difficult as Chinese.
'Less hard' or 'less difficult' has a slightly different meaning. It means a little less hard/difficult than current state. -You can make the ice cream less hard by letting it s
Yes, that is correct. Because you can't say "Chinese is difficulter than English."
Just remember, any word with 3 or more syllables requires you to use 'more' or 'most' instead of 'er' or 'est' to turn it into comparative or superlative form. This is a simple rule, but one that's easy to forget.