That man is Tom’s father, who/that is wearing a red shirt.

[diamondcutter]

That man is Tom’s father, who is wearing a red shirt.
(Made by me.)
I’d like to know whether the relative pronoun “who” could be replaced with “that”. I read somewhere (I can’t remember where) it’s acceptable in BrE. Is that true?

 

[Tarheel]

I don't think you can replace "who" with "that" in that sentence. (However, it doesn't much matter as you will never have a reason to use that one.)

 

[5jj]

That man is Tom’s father, who is wearing a red shirt.

That is a completely unnatural sentence.

 

[diamondcutter]

Would you tell me how to improve or correct it?

 

[5jj]

The man who is wearing a red shirt is Tom's father.

You could use 'that' in that defining relative clause, though many people prefer 'who'.

 

[diamondcutter]

Tom’s father, who/that is wearing a red shirt is under the tree.

What about this sentence? Does it make sense? And are both “who” and “that” acceptable in it?

 

[5jj]

Tom’s father, who/that is wearing a red shirt, is under the tree.

That works.

 

[diamondcutter]

The Thames, which/that is now clean enough to swim in, was polluted for over a hundred years.

Are both “which” and “that” acceptable in this sentence?

 

[5jj]

No.

'That' is not acceptable in non-defining relative clauses.

 

[Barque]

Would you tell me how to improve or correct it?

You could also say "The man in the red shirt is Tom's father".

Are both “which” and “that” acceptable in this sentence?

If you used "that", it'd mean there are multiple Thameses, and one of them is now clean enough to swim in. Of course most people wouldn't think of it that way as we know there's only one but that'd be the effect.