If you're a college student staring at a sentence diagram in your linguistics 101 class or a freshman composition workshop wondering why your professor circled "that" and wrote "function?" in the margin, you're not alone. The word "that" is a grammatical chameleon — it slips between roles so smoothly that even advanced students misidentify it. I've watched sophomore English majors at State University argue for twenty minutes over whether the "that" in "She said that he left" is a relative pronoun or a subordinating conjunction. The answer matters because it determines how you analyze the clause structure, which determines whether your syntactic tree gets full credit or gets sent back with red ink. Let's break down the decision process so you can stop guessing and start proving.
The Clause-Type Test: What Kind of Clause Does "That" Introduce?
The first factor to evaluate is the most visible one: what type of dependent clause follows "that"? This is where many students get tripped up because they've memorized that subordinating conjunctions create adverb clauses and relative pronouns create adjective or noun clauses — but they haven't internalized what those clause types actually do in a sentence. An adverb clause modifies a verb, adjective, or adverb in the main clause, answering questions like when, why, how, or under what condition. An adjective clause modifies a noun or pronoun. A noun clause functions as a subject, object, or complement. When you see "that" introducing a clause, ask yourself: what is this clause doing for the rest of the sentence?
Take the sentence "The book that I borrowed is overdue." The clause "that I borrowed" modifies "book" — it tells you which book. That's an adjective clause, so "that" is a relative pronoun. Now compare "I think that the book is overdue." The clause "that the book is overdue" is the direct object of "think" — it's the thing being thought. That's a noun clause, and "that" here is a subordinating conjunction. Notice that in both cases, "that" introduces a dependent clause, but the clause's function in the larger sentence is different. That's your first decision point.
Red flag: you label every "that"-clause as an adjective clause because you see a noun nearby. Green flag: you can name the clause's grammatical role — subject, direct object, object of preposition, subject complement, or modifier of a specific noun — and that role matches the clause type. Maria, a junior linguistics major at Midwest State, told me she used to highlight every "that" in her essays and guess. Then she started asking "what question does this clause answer?" If it answers "which one?" it's adjectival. If it answers "what?" after a verb like think, say, believe, or know, it's nominal. If it answers "why?" or "when?" or "how?" it's adverbial — though "that" rarely introduces adverb clauses; words like "because," "when," and "since" do that job. Her grades on syntax assignments jumped from C's to A's in one semester.
The Syntactic Role Test: What Job Does "That" Do Inside Its Own Clause?
The second factor goes deeper. Even when you've identified the clause type, you still need to know what "that" is doing inside that clause. This is where the distinction between form (part of speech) and function (grammatical role) becomes critical. A relative pronoun always has a syntactic function within its clause — it can be the subject, the direct object, the object of a preposition, or a possessive determiner. A subordinating conjunction has no syntactic function inside its clause; it's just a connector, a grammatical hinge that attaches the dependent clause to the main clause without playing a role in the dependent clause's internal structure.
Look at "The student that won the prize is my roommate." Inside the clause "that won the prize," the word "that" is the subject. It's doing the winning. You couldn't say "The student won the prize" and keep the same structure — you'd lose the relative clause entirely. Now look at "I heard that the student won the prize." Inside "that the student won the prize," the word "that" does nothing. The subject is "the student." The verb is "won." The object is "the prize." Remove "that" and you get "I heard the student won the prize" — still grammatical, still the same meaning. That's the hallmark of a subordinating conjunction: it's optional, functionally empty inside its clause, and serves only to mark subordination.
Red flag: you call "that" a relative pronoun in a noun clause because "it stands for something." Green flag: you can identify the subject, verb, and objects of the dependent clause and confirm whether "that" occupies one of those slots or sits outside them entirely. James, a first-year grad student in applied linguistics, shared that he used to confuse these because "that" looks the same in both positions. His breakthrough came when his advisor made him diagram five sentences a day for two weeks, forcing him to place "that" in the tree. Once he saw that relative pronouns get a theta role (subject, object) while subordinating conjunctions attach to CP (complementizer phrase) without a theta role, the distinction became physical, not abstract.
The Substitutability Test: Can You Swap "That" for a Wh-Word or a Clear Conjunction?
The third factor is a practical diagnostic you can run in your head during an exam or while editing a paper. Relative pronouns are part of the wh-family — who, whom, whose, which, that. If you can replace "that" with "who," "whom," "which," or "whose" without changing the meaning or grammaticality, you're almost certainly dealing with a relative pronoun. Subordinating conjunctions, by contrast, belong to a different lexical set — because, since, although, if, when, while. If you can replace "that" with one of those and the sentence still makes sense (even if the meaning shifts slightly), you're looking at a subordinating conjunction introducing an adverb clause — though remember, "that" itself rarely introduces adverb clauses in standard English. The real power of this test is distinguishing relative pronouns from subordinating conjunctions in noun clauses.
Consider "The reason that he gave was vague." Try "The reason which he gave was vague" — works perfectly. "That" is a relative pronoun, object of "gave." Now consider "The reason that he left was vague." Try "The reason which he left was vague" — fails. Try "The reason because he left was vague" — awkward but reveals the structure. Actually, "that he left" is a noun clause complement of "reason," and "that" is a subordinating conjunction. The substitutability test exposes the difference: relative pronouns have wh-alternates; subordinating conjunctions in noun clauses don't.
Red flag: you force a wh-substitution that changes the meaning ("The fact that he left" → "The fact which he left" — nonsense). Green flag: the substitution either works cleanly (relative pronoun) or fails cleanly (subordinating conjunction), and you recognize which pattern you're seeing. Priya, a senior English education major, told me she teaches this test to her tutoring students using color-coded index cards: blue for wh-words, red for subordinators. They sort example sentences into piles. It turns a vague intuition into a visible pattern.
The Omission Test: Can You Delete "That" Without Breaking the Sentence?
The fourth factor is the quickest check but also the most dangerous if used alone. In many dialects of English, "that" can be omitted when it's a subordinating conjunction introducing a noun clause after certain verbs (think, say, believe, know, hope) — "I think he's right" instead of "I think that he's right." It can also be omitted when it's a relative pronoun functioning as an object — "The book I borrowed" instead of "The book that I borrowed." But it cannot be omitted when it's a relative pronoun functioning as a subject — "The student won the prize" loses the relative clause entirely and becomes a simple sentence with a different structure. And it often cannot be omitted in formal writing even when grammatically possible, because the "that" provides clarity and rhythm.
This test is most useful as a tiebreaker. If you've narrowed it down to two possibilities and one allows omission while the other doesn't, try reading the sentence aloud without "that." If it sounds natural and the structure holds, you've got evidence. But never rely on this test alone — dialect variation, register, and verb class all affect omissibility. The omission test is a supporting witness, not the lead detective.
Red flag: you declare "that" a subordinating conjunction just because you can delete it in "The book I read." Green flag: you use omission as confirmatory evidence after the clause-type and syntactic-role tests have pointed you in a direction, and you note whether the register (formal academic vs. casual speech) affects the judgment.
| Factor | Weight | Go Threshold (Relative Pronoun) | No-Go Threshold (Subordinating Conjunction) |
|---|---|---|---|
| Clause-type test | 30% | Clause modifies a noun (adjective clause) or functions as subject/object/complement with "that" inside clause structure | Clause functions as noun clause complement with "that" outside clause structure |
| Syntactic role test | 35% | "That" serves as subject, object, object of preposition, or possessor inside its clause | "That" has no syntactic role inside clause; clause has its own subject/verb/object |
| Substitutability test | 20% | "That" replaces cleanly with who/whom/which/whose | "That" resists wh-substitution; may allow because/if/when in adverb clauses (rare for "that") |
| Omission test | 15% | Omission possible only if "that" is object relative pronoun; never if subject | Omission often possible after cognitive/communication verbs in noun clauses |
Still unsure? Here's what to do next: pick five sentences from your current reading — a textbook, a novel, a research article — that contain "that," and run each one through all four tests in order, writing down your evidence at each step. Don't just think it; write it. The act of recording "clause modifies 'book'" or "'that' is subject of 'won'" forces the analysis from intuition into explicit reasoning, and that's what your professors are actually grading. If you hit a sentence where the tests conflict, bring it to office hours with your written work — your instructor will see you've done the analytical heavy lifting and can target the exact point of confusion.