Indirect Speech: He said that she was a beautiful girl.
Direct Speech causes no problem to me but I always find Indirect Speech obscure and weird. Why can't it be said he said that she is a beautiful girl? If it were he said that she is a beautiful girl then I will be having no problem and that will make complete sense to me. Why can't is be used in place of was? What is the reason for using was?
You can say 'He said that she is a beautiful girl.' the meaning is that at the moment the sentence is being said, the situation referred to in that sentence is still true. She is still beautiful.
The past tense in the reported clause would normally imply that the situation in the sentence has changed. For example, 'He met her a long time ago and when I talked to him, he said that she was a beautiful girl at the time.'
It is not wrong to say, "She said that she is a beautiful girl." It emphasises that she is indeed a beautiful girl.
It is also not wrong to say, "She said that she was a beautiful girl.' This is because 'was' (a past tense verb) is consistent with the reporting verb 'said' (a past tense verb).
The teacher said, "The world is round" is usually reported as The teacher said that the world is round.
It is not wrong to say The teacher said that the world was round. ('was' is used to be consistent with 'said')