It's a very difficult question.

But I am going to answer it. (You don't technically need the "it" here but it sounds more natural to me)

It's funny!!! (Do you really mean it's funny or do you mean it's fun?)

I'm also happy when I get good marks, especially 5.

I am happy when it snows.

Then it's shining on the ground.

I am happy when I am glad. (lol)

Of course I should mention many more. (I think that's what you're trying to say)

But it's difficult to say.

It's a very difficult question.

ButHowever, I am going to answer it. ("but" is a conjunction that is normally placed in the middle of a sentence. What are you going to answer? It.)

So, I'm happy when my friends visit me.

Also, I have a am happy when I take get (or receive) good marks, especially 5's. ("marks" and 5's are both plural.)

Mark 2 is bad.=(

I am happy when it is snowing.

It's white, white.

Then, it shines on the earth (or ground).

I am happy when I am glad. (This sentence is tricky...You can say, "Being happy makes me glad," which is grammatically correct. However, it's redundant. You may use "I like to be happy" or "I like to be glad" instead. Who doesn't like to be happy? :))

Of course, I should saytalk about many more things (that make me happy),

but it's difficult as I said.