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What is the grammatical function of this that-clause?

In court, the witness took an oath that he would tell the whole truth.
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mehdi kord What is the grammatical function of this that-clause?
In court, the witness took an oath that he would tell the whole truth.

It's a content clause. It's the complement of the noun 'oath'.

(according to CGEL)

The verb analog is 'to swear': swore that he would tell the whole truth.

CJ

Comments  

It's adjectival on "oath". What kind of oath was it?

 CalifJim's reply was promoted to an answer.