That

That is a function word used in the English language for several grammatical purposes.

These include:

• as a complementizer/subordinating conjunction. ("He asked that she go.") ("That" is not used this way in American and Canadian English.)
• 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 a 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." 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.

https://en.wikipedia.org/wiki/That