I was wandering (wondering) if

Quick answer

Match the verb to the head noun (the noun immediately before the relative pronoun who/which/that), not to any nouns inside the relative clause. If the head noun is singular, use singular verbs (is, was, has); if it's plural, use plural verbs (are, were, have).

Find the head noun: in "The book that the students read was long," the head noun is "book" (singular).
Ignore intervening nouns inside the clause: "students" does not change the verb form.
Quick check: remove the relative clause. If "The book was long" sounds right, the verb is correct.

Is "The book that" correct?

"The book that" is grammatically correct when the head noun is singular and the clause describes it. Problems arise when writers let plural nouns inside the clause pull the verb away from the head noun.

Example: "The book that the students reviewed was long." The verb is "was" because the head noun "book" is singular even though "students" is plural.

Spacing and hyphenation - or something else?

Some errors look like subject-verb mistakes but stem from spacing, hyphenation, or broken word forms. Those are separate issues but often appear near each other in drafts.

Check whether a phrase should be one word, hyphenated, or two words; that clarifies whether you have a noun that needs a singular or plural verb.
Example: "backup plan" vs. "back-up plan" doesn't change agreement, but inconsistent spelling can distract you from the real subject-verb problem.
If a word looks unfamiliar, rewrite the sentence to expose the true head noun and verb.

Why writers make this mistake

Writers often match the verb to the nearest noun or the most salient noun inside the clause, not to the head noun. Fast drafting and speech-based spelling can reinforce that error.

Attraction to a nearby plural (proximity attraction)
Complex noun phrases or long intervening clauses
Relying on how a sentence sounds rather than testing the head noun + verb
Typos and inconsistent spacing that hide the real subject

How it sounds in real writing

Seeing short, natural examples helps you hear the right forms.

Work: The report that the team submitted was thorough. (head noun "report" → "was")
Work: The data that the analysts collected are available in the dashboard. (head noun "data" → "are")
Work: The list that I sent you contains the priorities for Friday. (head noun "list" → "contains")
School: The essay that the students wrote drew strong feedback. (head noun "essay" → "drew")
School: The criteria that the committee set are strict but fair. (head noun "criteria" → "are")
School: The chapter that I assigned last week is required reading. (head noun "chapter" → "is")
Casual: The bag that you found was mine. (head noun "bag" → "was")
Casual: The pictures that we took are on my phone. (head noun "pictures" → "are")
Casual: The plan that he suggested sounds promising. (head noun "plan" → "sounds")

Wrong vs right examples you can copy

Six quick pairs that show common relative-clause agreement errors and their fixes.

Wrong: The book that the students read were heavy.
Right: The book that the students read was heavy.
Wrong: The group that attends every meeting is bringing snacks.
Right: The group that attends every meeting is bringing snacks. (If you mean the members, you could write: The members of the group who attend every meeting are bringing snacks.)
Wrong: The list that I made contain three tasks.
Right: The list that I made contains three tasks.
Wrong: The problems that the engineers fixed was complex.
Right: The problems that the engineers fixed were complex.
Wrong: The box that my friends sent were full of books.
Right: The box that my friends sent was full of books.
Wrong: The rules that everyone agrees on is listed below.
Right: The rules that everyone agrees on are listed below.

How to fix your own sentence

Follow a short process that exposes the head noun and forces the correct verb.

Step 1: Identify the head noun immediately before who/which/that.
Step 2: Remove the relative clause and read the subject + verb alone.
Step 3: If it sounds wrong, change the verb to agree with the head noun or rewrite the sentence to make the subject clearer.

Three rewrite patterns you can copy:

Original: The book that you and I bought were on sale.
Rewrite: The book we bought was on sale.
Original: The committee that interview candidates are meeting tomorrow.
Rewrite: The committee that interviews candidates is meeting tomorrow.
Original: The students who missed the exam was upset.
Rewrite: The students who missed the exam were upset.

A simple memory trick

Visualize the head noun and its verb as a pair and temporarily hide the whole relative clause. If "head noun + verb" sounds correct, the sentence is probably correct.

Practice by scanning for who/which/that and reading only the words before and after it.
Search your drafts for common relative pronouns and fix them in bulk.

Similar mistakes to watch for

Spotting related errors nearby saves time when you proofread.

Agreement with compound subjects ("Tom and Jerry are" vs. "Bread and butter is")
Collective nouns that shift between singular and plural depending on meaning
Subject confusion after introductory phrases ("There is" vs. "There are")
Broken words or spacing that hide the true subject

FAQ

Should it be "The book that I borrowed was interesting" or "were interesting"?

Use "was" because the head noun "book" is singular. Ignore plural nouns inside the clause; the verb agrees with the head noun.

How do I handle collective nouns like "team" - is it "is" or "are"?

In American English, treat most collective nouns as singular: "The team that trains together is improving." In British English, you may see plural verbs when the group members are emphasized: "The team are arguing among themselves."

Can commas change which verb I should use?

No. Commas mark nonrestrictive (extra) clauses but do not change which noun is the head. They affect meaning but not agreement: "My brother, who lives in Boston, is visiting" - "brother" is singular, so "is" stays singular.

What's a quick way to fix "The book that you and I bought were on sale"?

Remove the clause: "The book was on sale." Or rewrite to tighten the clause: "The book we bought was on sale." Both make the subject clearly singular.

Are grammar checkers reliable for these errors?

Grammar checkers catch many agreement errors and offer useful suggestions, but always apply the "remove the clause" test yourself-tools can miss subtle context or suggest awkward rewrites.

Quick habit to reduce errors

Before sending an email or submitting a draft, pause and read aloud only the head noun + verb. If it sounds mismatched, remove the clause and either correct the verb or rewrite the sentence. This habit catches most relative-clause agreement mistakes.