fFarewell

The same face, the same person, lying quietly in front of us, but the only difference is that he didoesn't speak, and didn't nor breathe anymore.

The world is a stage, the same land, the same sky, and the same air, but the difference is that there are different people on the stage at different times.

Everybody will appear on the stage and they will definitely they will also step off the stage, and they will never come back again.

I really don't know!

You can also say ," I don't really know!".

Being alive, let's always be happy because we don't know which day we will go with the wind.

fThe Farewell

You should place an article in front of farewell, otherwise, it sounds like you are saying farewell to someone. Another option is to change the title to A Farewell to my Old Classmate.

The same face, the same person, lying quietly in front of us, but the only difference is that he didoesn't speak, and didoesn't breathe anymore.

The world is a stage, with the same land, the same sky, and the same air, but the difference is the differenat people appear on theis stage at different times.

Everybody wiWe will all appear on theis stage and definitely they will also step off the stage, and they willand never come back again.

Based on your first sentence, I'm keeping the story in the same tense (ie: you used "us", so you should not switch over to "they"). Also, what you wrote was technically correct, but sounded heavy. A good rule of thumb when writing, according to Hemingway, is that one should never use a word that serves no purpose.

What' is life?

I would not use a contraction when you are posing a serious question in your story. You want to emphasize that you are about to explain it to the readers.

Being alive, let'sWhen we are alive, we should always be happy because we don't know which dayen we will go with the wind.

The way you wrote this sentence, "being alive" is a dangling participle. What about being alive? you did not finish your thought before you started a different idea of being happy.

Old classmate, Rmay you rest Iin Ppeace.

The same face, the same person, lying quietly in front of us, but the only difference is that he didoesn't speak, and didn't or breathe anymore.

The world is a stage,: the same land, the same sky, and the same air, but the difference is that there are different people on the stage at different times.

What' is life?

Sounds more natural without the contraction.

BeingSince we are alive now, let's alwaystry to be happy, because we don't know which day we willen our time will come to go with the wind.

I just tried to make it sound more natural.

OldFormer classmate, Rest In Peace.