The word that works differently depending on how you use it in a sentence.

Most commonly, that points to something specific, usually something farther away or something you just mentioned. You might say “I want that cookie” while pointing across the table, or “I read that book last summer” when referring to one you already talked about. It helps identify which exact thing you mean out of several possibilities.

That also connects ideas within sentences. When you say “I know that the test is tomorrow,” the word that links your knowledge to the specific fact about the test. You could often leave it out (“I know the test is tomorrow”), but including it can make your meaning clearer, especially in writing.

Sometimes that introduces extra information about something: “The student that won the spelling bee studies every night.” Here it tells you which student by adding a description. You'll also hear people use who instead in this situation, and both work fine when talking about people.

Finally, that can show how much or to what extent: “I'm not that hungry” or “It wasn't that difficult.” This usage emphasizes the degree of something, often suggesting it's less than someone might think.