I made the changes you wrote. Can you please check if everything is OK?

1) The rooms are all set in a way that one cannot see from one room to another; besides, each room has a Gothic window and ornaments of the same colour.
2) Only the last one is all black with a red window. All the guests are enjoying themselves and dancing. However, when a clock with a pendulum strikes the hours, the orchestra stops playing and the people stop dancing, as if they were all frozen.
3) They all resume their activities as soon as the clock finishes chiming. When the clock strikes midnight, a man with a red costume turns up.
4) He is covered in blood and wears a mask on his face that makes him similar to a dead man. The man moves to and fro with solemn movements.
5) When Prince Prospero tries to stop him, he immediately dies. The same happens to his friends who try to touch him but realize there is no tangible form under the costume.
6) One by one, all the people in the castle die, and darkness falls all over the place.

He likes teaching adults.
He likes taching German to adults.
He likes teaching adults German.

Which are possible?

Good corrections! Nice! Only a couple more that I see now:

4. ... wears a mask that makes him look like a dead man.

5. ... happens to Prospero's friends who try to touch the dead man, but realize...

All three of the last sentences are fine. They have slightly different phrasing, but they are all grammatically correct.

PS - Since masks are worn ONLY on a person's face, there's no need to state that in the sentence.

Based on the changes you made, let's review each sentence:

1) The sentence seems fine grammatically. It describes the rooms and the features they have.
2) The sentence is grammatically correct and provides details about the behavior of the guests and the impact of the clock striking the hours.
3) The sentence is grammatically correct and introduces a new character, a man in a red costume appearing at midnight.
4) The sentence is grammatically correct and describes the appearance and movements of the man in the red costume.
5) The sentence is grammatically correct. It describes the consequences of trying to interact with the man in the red costume.
6) The sentence is grammatically correct and indicates the progression of events leading to the demise of all the people in the castle.

As for the second part of your question, which sentences are possible, it depends on the intended meaning:

1) "He likes teaching adults." - This sentence is a general statement that he enjoys teaching adults, but it does not specify what subject or language he teaches.
2) "He likes teaching German to adults." - This sentence specifies that he enjoys teaching the German language to adults.
3) "He likes teaching adults German." - This sentence also specifies that he teaches the German language to adults, but it emphasizes the fact that he teaches German rather than any other subject.

All three sentences are grammatically correct, but they convey slightly different meanings.