In this example, "The more tired you are, the harder you concentrate" should actually be "The more tired you are, the harder it is to concentrate" because the way it was written above means that when you are tired, you concentrate more, which to me does not make much sense since when I am tired I want to sleep haha. Anyways, regarding your question..in this sentence pattern, you can use both "you" and "it is to"; it will just depend on the sentence. For example, I can say "The harder you study, the more you learn" but another sentence can say "The more you know, the harder it is." There isn't really a specific patter here..you just have to go with whichever sounds right ^^

The meaning is : the more tired you are, It is harder to concentrate. Your sentence means: the more tired you are the harder you concentrate. The first one shoes that it is more difficult to do. The second shows that you do something more.

hardは品詞によって意味が変わるのですが、"the harder you concentrate"は"you concentrate hard"みたいな文をひっくり返したもので、このhardは副詞で、意味は「熱心に」「懸命に」ってところでしょうか。 " It is harder to concentrate."のhardは形容詞で、意味は「難しい」ですよね。 つまり、文法的にはどちらも正しいわけですが、「疲れれば疲れるほど、懸命に集中するようになる」ってのはあんまりしっくりいかないので、意味の上から正解ではないわけです。

Is "the harder you can concentrate it" also okay? Wait wait, I'm a little confused again

I think you can also say: the more tired you are, the harder you can concentrate. "It" indicates the difficulty of the concentration.