She looked at his eyes that were just beautiful
I know that this sentence is wrong but I don't know how to correct it. Do I have to change 'that' into 'which'?
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I know that this sentence is wrong but I don't know how to correct it. Do I have to change 'that' into 'which'?
Yes, change "that" into "which" and use a comma. By writing "she looked into his eyes that were" technically you are suggesting he has many eyes and she is only looking at the ones that are beautiful, though no one would be confused about what you meant to say.
The right way to write this is
She looked at his eyes, which were just beautiful.
Stylistically, if he's looking back (she's not looking at a picture), normally we would say
She looked into his eyes, which were just beautiful.
A more straightforward rewrite would be
She looked into his beautiful eyes.
Original: She looked at his eyes that were just beautiful.
I will assume we are talking about people.
Unless she is talking to a man who has one or more eyes on the back of his head, in addition to the pair on his face, I think that "that" does not work here. Usually, "that" is used when the clause that follows the "that" is restrictive/essential.
A restrictive clause is a part of a sentence that may not be removed without rendering that sentence incomplete, difficult to understand, or with its meaning substantially changed. - Merriam-Webster Dictionary of English Usage (MW)
In your sentence, "...were just beautiful" is not essential to understand what came before:
She looked at his eyes.
Removing the additional information that his eyes were beautiful does not change the primary meaning of the sentence - She looked at his eyes. Now you add in important information (important but not essential):
She looked at his eyes, which were just beautiful.
What follows after "which" is a nonrestrictive/nonessential clause.
[A] nonrestrictive clause adds information to the sentence, but it is not imperative to include it. - MW
You can use either which or that to introduce a restrictive clause—the grounds for your choice should be stylistic—and which to introduce a nonrestrictive clause. - MW
Other resources -
-When to use “that” and when to use “which”?