TOEFL Complete the Words: Word Families and Affixes

Suppose the visible letters are subst______ in this sentence:

Restoring the wetland produced a subst______ increase in nesting sites.

You may recognize the family substance, substantial, and substantially. Only substantial fits. The blank sits before a noun, so it needs an adjective; a substantial increase is a natural pattern; and the passage describes a large change.

This is different from merely knowing what substantial means. TOEFL Complete the Words asks you to rebuild partially missing words inside a connected text. ETS’s current overview says the task uses an academic passage of about 70–100 words. After an intact first sentence, the second half of every second word is removed, creating ten incomplete words. ETS describes the task as processing text for meaning and form.

The broader TOEFL Complete the Words vocabulary-in-context guide teaches a four-filter method: visible form, syntax, collocation, and passage meaning. This article adds a narrower skill: using word families and affixes without letting them override the sentence.

Find more practice at the TOEFL vocabulary practice hub and Exam English hub.

Read both sides of a word

The visible beginning narrows the family. The sentence predicts the ending.

For each incomplete word, ask:

1. What letters are fixed?
2. What part of speech belongs in the slot?
3. Which family member has the right ending and spelling?
4. Does that member form a natural phrase with nearby words?
5. Does it preserve the passage’s claim strength?

Affixes are clues, not answers. The ending -ly often marks an adverb, but not every word ending in -ly is an adverb. The ending -tion often marks a noun, yet the sentence may need a different noun in the same family. Always return to grammar and meaning.

1. Prevalent: adjective after a linking verb

Prevalent means widespread or commonly occurring within a place, group, or period.

The practice remains prevalent among coastal communities.

The pattern prevalent among + group or prevalent in + place or field helps identify both form and meaning. After remains, the sentence needs an adjective. Prevalence is the noun: “The study measured the prevalence of the practice.”

Do not confuse prevalent with predominant. Predominant highlights being the main or strongest among alternatives. A custom can be prevalent—common across many communities—without being the predominant custom.

Family check:

• prevalent: adjective — “is prevalent among…”
• prevalence: noun — “the prevalence of…”
• prevail: verb with different patterns — “the view prevailed”

Shared letters do not make the forms interchangeable.

2. Substantial and substantially: choose the slot

Substantial means large or important in amount, degree, or extent.

Researchers observed a substantial reduction in water loss.

Substantially is the adverb:

Water loss was substantially lower after the change.

The neighboring words decide. Before the noun reduction, use the adjective. Before the comparative adjective lower, use the adverb. Neither form adds certainty, permanence, or sole causation.

Words such as considerable and significant may overlap in some contexts. Do not rely on a memorized ranking in which substantial is always stronger. In a partial-word task, the visible letters, grammatical slot, and exact passage—not a synonym ladder—must resolve the answer.

3. Negligible: small enough not to matter here

Negligible describes an amount, difference, cost, risk, or effect so small that it is not worth considering in the decision at hand.

The temperature difference was negligible, so it did not change the classification.

The ending -ible identifies an adjective, but meaning still matters. A small difference is not negligible if the passage says it changed the conclusion. The word makes a contextual judgment about importance.

Keep the family boundary clear:

• negligible: too small to matter in context
• negligence: failure to take proper care
• negligent: careless

Their spelling overlaps, but their meanings and sentence roles do not. Do not choose negligible merely because negli_____ is visible; test the claim against the whole paragraph.

4. Conducive to: a fixed pattern and a calibrated claim

Conducive to means tending to promote or provide suitable conditions for a result.

Stable soil moisture is conducive to seed germination.

The fixed preposition is to, followed by a noun or gerund: conducive to growth, conducive to learning, conducive to reducing waste. Not conducive for growth in the taught pattern, and not conducive to reduce.

Do not confuse conducive with conductive, which describes the capacity to conduct heat or electricity. Also, conducive to does not prove that the outcome occurred. Suitable conditions may promote germination without guaranteeing that every seed germinated.

This item tests more than an affix. It combines spelling, the adjective slot after is, the fixed preposition, and claim strength.

Use suffixes as predictions

Before reconstructing a word, identify the sentence frame:

These are useful predictions, not universal laws. English permits many structures. Read enough of the sentence to know what the blank actually modifies.

Worked word-family passage

The following original passage contains ten practice gaps. It imitates the task’s reconstruction demand but is not an official TOEFL item.

Urban wetlands can store storm water, support wildlife, and reduce heat, although their effects depend on size, location, vegetation, and long-term maintenance; these ecosystems can also filter pollutants before runoff reaches nearby rivers and reservoirs. In den___ districts, floo____ may b_ prevalent wh__ paved surf____ prevent run___ from ente____ the so__. Restoration acr___ a subst______ area can create conditions conducive to recovery, whereas a very small isolated site may produce only a negligible change.

Answers

1. den___ → denser. The comparative adjective modifies districts.
2. floo____ → flooding. The clause needs a noun subject before may be.
3. b_ → be. The modal may requires the base form.
4. wh__ → when. The conjunction introduces the condition involving paved surfaces.
5. surf____ → surfaces. A plural noun agrees with prevent.
6. run___ → runoff. The verb prevent needs an object before from.
7. ente____ → entering. Prevent X from + -ing controls the form.
8. so__ → soil. The sentence needs the noun receiving the runoff.
9. acr___ → across. The preposition connects restoration with its spatial extent.
10. subst______ → substantial. The slot before area requires an adjective meaning large in amount or extent.

Notice that the advanced expressions are not artificially selected as the ten gaps. The alternating deletion pattern determines which words are incomplete. Substantial, conducive to, and negligible remain intact and help establish meaning for the ordinary missing words.

That is important practice. A real text will not conveniently hide only the vocabulary you studied. Familiar function words, inflections, and common academic forms may carry the gaps.

A second family decision

Consider this mini-context:

The intervention produced a subst______ improvement. Performance was subst________ higher after six weeks.

The first answer is substantial because it modifies improvement. The second is substantially because it modifies higher. Meaning alone cannot distinguish them; syntax does.

Now consider:

The new lighting reduced energy use by 2%, but that difference determined whether the building met the legal threshold.

Would negligible be justified? No. Even if 2% looks numerically small, the passage says it changed a consequential classification. The paragraph overrides your intuition about the number.

Common mistakes

• Completing a word from its prefix before reading the slot.
• Treating every -ly ending as an adverb or every -tion ending as the required noun.
• Choosing the correct family but the wrong member: substantial for substantially.
• Ignoring inflection: singular/plural, tense, agreement, or -ing after a preposition.
• Confusing related spellings with related meanings, such as negligible and negligent.
• Breaking fixed patterns: conducive for or conducive to reduce.
• Assuming a sophisticated completion is better than an ordinary one.
• Solving each sentence alone when a contrast or cause in the next sentence changes the meaning.

Quick check

1. “The behavior was increasingly prev_____ among the sampled birds.” Which form fits: prevalent or prevalence?
2. “The change was subst________ greater in shaded plots.” Which form fits?
3. “The difference was small but changed the safety decision.” Can it accurately be called negligible?
4. Complete the pattern: “Quiet conditions were conducive ___ sustained attention.”
5. Which spelling describes material that carries heat: conducive or conductive?

Answers: 1. Prevalent; the slot after was needs an adjective. 2. Substantially; the adverb modifies greater. 3. No; the difference mattered to the stated decision. 4. to. 5. Conductive; conducive to means tending to promote a result.

Build your own reconstruction drill

Write an original 70–100-word academic paragraph. Keep the first sentence intact. Beginning with the second sentence, delete the second half of every second word until you have ten gaps. Do not select only difficult content words.

For each answer, record:

• fixed letters and required spelling;
• part of speech and inflection;
• nearby collocation or grammatical frame;
• passage-level evidence;
• one family member that fails and the exact reason it fails.

If the failed alternative remains grammatically and semantically possible, the item is ambiguous. Revise the passage rather than inventing a rigid distinction.

Sources

• TOEFL iBT Reading section
• Updated TOEFL iBT Test Overview
• Inside the TOEFL iBT updates: validity by design

Related

• TOEFL Complete the Words: vocabulary in context
• TOEFL Build a Sentence: collocations that fit
• TOEFL Read an Academic Passage: claims and evidence
• Why a TOEFL word list will not raise your writing score

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