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- as a complementizer/subordinating conjunction. ("He asked that she go.")
- to introduce a restrictive relative clause ("The test that she took was hard.") In this role, that may be analyzed either as a relative pronoun or as a conjunction as in the first case; see English relative clauses: That as relativizer instead of relative pronoun. (In American and Canadian English, "that" is only used in this way if the verb could affect the proceeding noun, i.e., one would say "The test she took was hard," but would still say "I hate that dogs are messy" to avoid being misheard as saying "I hate dogs.")
- as a demonstrative pronoun ("That was hard.") (plural: those)
- as a demonstrative adjective ("That test was hard.") (plural: those)
- as an adverb ("The test wasn't that bad.")
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 edition 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."
The word "that" are in the formation of the restrictive relative clause, especially one identification. These words are used to modify an adjective, a noun, and a pronoun, for a specify information about things, subject and object, if replaced by It. (e.g.: -s The one that works well, -ed The one that worked well.)
Also, the word "that" is used in subordinate conjunctions describing a person or people. In demonstrative, "that" is singular, and "those" is plural; e.g. "That is the bat" (singular), and "Those are the bats" (plural).