Are the following sentences correct?

He said it was them who took it.,
It couldn't have been them.,
Elena and she tied for first place.

The first two sentences are incorrect.

The third one is correct.

http://grammar.ccc.commnet.edu/grammar/cases.htm

To determine if the given sentences are correct, we can analyze each sentence individually:

1. "He said it was them who took it."
In this sentence, the use of "them" is incorrect. The correct pronoun to use would be "they." The sentence should be revised to: "He said it was they who took it."

To explain how to get to the correct answer:
To determine whether to use "them" or "they," you can break down the sentence and identify the subject and verb of the embedded clause. In this sentence, the subject of the embedded clause is "they." Therefore, "they" should be used instead of "them."

2. "It couldn't have been them."
This sentence is grammatically correct. The pronoun "them" is used correctly here.

To explain how to get to the correct answer:
To determine if "them" is the correct pronoun, you can consider its function in the sentence. In this case, "them" is serving as the object of the verb "been."

3. "Elena and she tied for first place."
This sentence is incorrect. The pronoun "she" should be replaced with the pronoun "her." The sentence should be revised to: "Elena and her tied for first place."

To explain how to get to the correct answer:
To choose between "she" and "her," you can consider the function of the pronoun in the sentence. In this case, "Elena and her" is the object of the verb "tied." Therefore, the objective case pronoun "her" should be used instead of the subjective case pronoun "she."