He bought one that work (works) underwater

Sentences such as "He bought one that work underwater" fail because the verb in the relative clause must agree with the head noun-here, one → works. Focus on the true subject, not a nearby plural inside a modifier.

Below are concise rules, a quick diagnostic, many labeled wrong/right examples (work, school, casual), and simple rewrites you can use immediately.

Quick answer

If the head subject is singular (one, each, everyone, someone, the only student), use a singular verb in the relative clause: one that works, everyone who knows. Ignore plural nouns inside modifiers (of the students, with the tools) when choosing the verb. If meaning is unclear, rewrite.

Find the head noun or pronoun, not a noun inside a modifier.
Treat indefinite pronouns (everyone, someone, nobody) as singular.
When a construction is ambiguous (one of the X that ...), rewrite to clarify.

Core explanation: why "one that work" is wrong

The verb must agree with the head of the noun phrase. In "one that work," the head is one (singular), so the correct verb is works. Matching a verb to a nearby plural inside a modifier produces errors or a change in meaning.

Rule: Verb agrees with the main subject (head), not with nouns inside modifiers.
Special case: "one of the + plural + that" can be ambiguous; meaning decides singular vs. plural verb.

Wrong: He bought one that work underwater.
Right: He bought one that works underwater.
School - Wrong: She's the only student who know the answer.
School - Right: She's the only student who knows the answer.

Common grammar patterns to watch

Most mistakes come from relative clauses after one/each/every, "one of the ...", indefinite pronouns, and subjects separated by prepositional phrases.

One/each/every + singular head → singular verb: one that works; every student knows.
Indefinite pronouns (everyone, someone, nobody) → singular verb: Everyone is ready.
"One of the + plural": decide whether the clause modifies the group (plural) or the single item (singular).
Ignore prepositional phrases (of the students, with the dogs) when selecting the verb.

Usage: One of the reports that is on my desk needs editing. (Clause pinpoints a single report.)
Usage: One of the reports that are on my desk needs editing. (Clause describes several reports; meaning differs.)
School - Wrong: Everyone that enjoy the class are welcome.
School - Right: Everyone who enjoys the class is welcome.

Fix your sentence: a 3-step diagnostic and quick rewrites

Find the true subject, remove modifiers, then match verb number. If still unclear, rewrite for clarity.

Step 1: Ask who or what performs the action - that is the subject.
Step 2: Remove modifiers and test subject + verb alone.
Step 3: If ambiguous, rewrite (e.g., "one of the X that..." → "a X that..." or pluralize).

Wrong: One of the reasons that make me nervous is the deadline.
Right: One reason that makes me nervous is the deadline.
Rewrite:
Original: "One of the students who missed the lecture were disciplined." →
Rewrite: "One student who missed the lecture was disciplined."
Rewrite:
Original: "One of the players that score often was injured." →
Rewrite: "One of the players who scores often was injured."

Examples: wrong/right pairs by context (work, school, casual)

Each wrong sentence shows a common trap; the corrected sentence fixes agreement or removes ambiguity.

Work - Wrong: The new tool we ordered are one that improve productivity.
Work - Right: The new tool we ordered is one that improves productivity.
Work - Wrong: I need a machine that operate 24/7.
Work - Right: I need a machine that operates 24/7.
Work - Wrong: They found one of the reports that needs more data are on the server.
Work - Right: They found one of the reports that needs more data was on the server.
Work - Rewrite: "One of the reports that needs more data was on the server." → clearer: "A report that needs more data was on the server."
School - Wrong: Everyone who take the exam must bring a pencil.
School - Right: Everyone who takes the exam must bring a pencil.
School - Wrong: She's the only student that were prepared on time.
School - Right: She's the only student who was prepared on time.
School - Wrong: One of the students who is absent are on leave.
School - Right: One of the students who is absent is on leave. (Or: One of the students who are absent is on leave - meaning differs.)
Casual - Wrong: I want something that taste like lemon.
Casual - Right: I want something that tastes like lemon.
Casual - Wrong: He bought one that look really cool.
Casual - Right: He bought one that looks really cool.
Casual - Wrong: They were the only ones that agree with him.
Casual - Right: They were the only ones who agreed with him.
Rewrite:
Original: "I want a phone that last for days." →
Rewrite: "I want a phone that lasts for days."
Rewrite:
Original: "I need one that hold many files." → Short fix: "I need a model that holds many files."

Real usage and tone: who vs that and meaning shifts

Use who for people, that (or which in some cases) for things. Agreement still follows the head noun. Small verb changes can shift meaning in "one of the + plural + that" constructions.

"Who" for people; "that" or "which" for things in restrictive clauses.
Meaning change example: "One of the teachers who work late got a raise." (several teachers work late) vs "One of the teachers who works late got a raise." (one teacher works late).
Casual - Wrong: Someone that know the password should log in.
Casual - Right: Someone who knows the password should log in.

Try your own sentence

Test the whole sentence - context often resolves the correct verb. Use the three-step diagnostic when a checker flags a potential issue.

Hyphenation and punctuation: clarity aids agreement checks

Hyphens and punctuation don't change agreement, but they help you spot the head noun. Group long modifier chains so the subject stands out.

Use hyphens in compound adjectives before a noun so the head noun is easier to find: one-of-a-kind phone → verb agrees with phone.
Punctuation (commas, parentheses) separates modifiers; mentally remove them to test the subject and verb.
Hyphens don't change number: "a one-off decision that affects..." → verb agrees with decision.

Usage: Correct: "We bought a one-of-a-kind chair that fits the space." The verb "fits" agrees with chair.
Work - Wrong: "He ordered a decision-making tool that improve workflows."
Work - Right: "He ordered a decision-making tool that improves workflows."

Spacing and formatting tips for proofreading agreement

Formatting doesn't alter grammar, but simple edits make errors jump out when you proofread.

Read sentences aloud to hear singular/plural mismatches.
Temporarily bold or mark the subject and verb: One | that work → you'll see the mismatch.
Remove modifiers on a copy and read the skeleton: "One works" vs "One work."

Edit tip: Highlight subject and verb - mismatches become obvious.
Proofreading trick: Delete prepositional phrases and test the core clause.

Memory tricks and quick fixes

Use short mental shortcuts or simple rewrites when you need a fast, reliable fix.

Mnemonic: "Head first, modifier later." Find the head noun before picking the verb.
If the head is one/each/every/everyone → verb usually ends in -s (works, knows, lasts).
Quick rewrites: "one that" → "a model that"; "one of the X that..." → "a X that..." or pluralize the subject.

Rewrite: "I need one that hold many files." → "I need a model that holds many files."
Rewrite: "One reason that make it hard..." → "One reason that makes it hard is..."
Usage: Mentally remove modifiers: "One of the reasons..." → try "One reason..." to choose the verb.

Similar mistakes to check next

After fixing "one that" problems, check agreement with joined subjects, collective nouns, and pronouns like everyone/they - these can follow different rules or depend on meaning.

Subjects joined by and → usually plural: The manager and the assistant are present.
Subjects joined by or/nor → verb agrees with the nearer subject: Either the students or the teacher is responsible.
Collective nouns (team, committee) take singular or plural verbs depending on whether the group or the members are meant.

Wrong: Neither the manager nor the employees was available.
Right: Neither the manager nor the employees were available.
Work - Wrong: The team are ready to present their findings.
Work - Right: The team is ready to present its findings. (Or: The team members are ready to present their findings.)

FAQ

Is "one that work" ever correct?

Not when the antecedent is the singular pronoun one. Use "one that works." In sentences like "one of the things that work," a plural verb can be correct if the clause describes the plural noun (things). When uncertain, rewrite.

Should I use "that" or "who" after "one"?

Use who for people and that (or which in non-restrictive contexts) for things: "one who knows" (person) vs "one that works" (thing). Agreement follows the head noun in either case.

How do I check a sentence quickly for agreement?

Find the subject (ask who/what), remove modifiers and prepositional phrases, and test the subject with the verb alone. If meaning remains ambiguous, rewrite the sentence simply.

What's the rule with "one of the + plural + that"?

If the relative clause modifies the plural group, the verb can be plural: "One of the students who are late will miss the test" (several students are late). If the clause restricts a single item, use a singular verb. When in doubt, rephrase.

Can a grammar checker always catch these errors?

Checkers catch many errors but can miss cases where meaning determines the correct verb. Use a checker, then apply the three-step diagnostic and rewrite if the sentence could mean more than one thing.

Want to check a sentence right now?

Copy a sentence into an editor, run a grammar checker, then use the three-step diagnostic above to confirm the fix. When in doubt, prefer a simple rewrite-clarity beats a clever but confusing structure.