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The function word That, is used on English language for several grammatical purposed .
In the first two uses the word is usually pronounced weakly, as /ðət/, whereas in the other uses it is pronounced /ðæt/.
In the Old English language that was spelled þæt. It was also abbreviated as a letter Thorn, þ, with the ascender crossed, ꝥ ( ). In Middle English, the letter Ash, æ, was replaced with the letter a, so that that was spelled þat, or sometimes þet. The ascender of the þ was reduced (making it similar to the Old English letter Wynn, ƿ), which necessitated writing a small t above the letter to abbreviate the word that ( ). In later Middle English and Early Modern English the þ evolved into a y shape, so that the word was spelled yat (although the spelling with a th replacing the þ was starting to become more popular) and the abbreviation for that was a y with a small t above it ( ). This abbreviation can still be seen in reprints of the 1611 ion of the King James Version of the Bible in places such as 2 Corinthians 13:7.
That is often omitted when used to introduce a subordinate clause—"He told me that it is a good read." could just as easily be "He told me it is a good read." Historically, "that" usually followed a comma: "He told me, that it is a good read." Middle Modern English grammarian Joseph Robertson recommended in On Punctuation that a comma be used with a conjunction. However, if the subordinate, conjunctional ellipse, null complement, or syntactic pleonasm of "that" is punctuated with a comma, then, in the English grammar, stylistically speaking, it is a comma splice, especially in formal writing. Instead, a semicolon should be used to be grammatically correct: He told me; it is a good read. In grammar, the usage of "that" constitutes a that-clause while its absence constitutes a bare clause.