Prefixes

A prefix is a group of letters added to the beginning of a word to change its meaning. In P3, you learnt about prefixes like un-, re-, pre-, mis-, and dis-. Now you will learn more prefixes that change meanings in interesting ways.

What You'll Learn

The negative prefixes in-, im-, ir-, and il-, and when to use each one
The prefixes over- and under-, which show "too much" or "too little"
How to recognise prefixed words and work out their base meanings
How the spelling of the base word determines which negative prefix to use

When to Use

Making a word mean "not" or "opposite": "It is impossible to finish this in one minute." (im- + possible = not possible)
Showing something is too much: "Be careful not to overcook the noodles." (over- + cook = cook too much)
Showing something is too little: "The hawker undercharged me by two dollars." (under- + charged = charged too little)
Describing something that is not correct or not complete: "His handwriting was illegible, so the teacher could not read it." (il- + legible = not legible)
Describing something that does not follow a pattern: "The bus arrived at irregular times today." (ir- + regular = not regular)

How to Form

Negative Prefixes: in-, im-, ir-, il-

These four prefixes all mean "not". The one you use depends on the first letter of the base word.

Prefix	Used Before	Base Word	Prefixed Word	Meaning
in-	most consonants	correct	incorrect	not correct
in-	most consonants	complete	incomplete	not complete
in-	most consonants	visible	invisible	not visible
in-	most consonants	dependent	independent	not dependent
im-	words starting with m or p	possible	impossible	not possible
im-	words starting with m or p	mature	immature	not mature
im-	words starting with m or p	patient	impatient	not patient
im-	words starting with m or p	polite	impolite	not polite
ir-	words starting with r	regular	irregular	not regular
ir-	words starting with r	responsible	irresponsible	not responsible
il-	words starting with l	legal	illegal	not legal
il-	words starting with l	legible	illegible	not legible
il-	words starting with l	logical	illogical	not logical

Over- and Under- Prefixes

These prefixes show amount -- too much or too little.

Prefix	Meaning	Base Word	Prefixed Word	Meaning
over-	too much	cook	overcook	cook too much
over-	too much	sleep	oversleep	sleep too long
over-	too much	crowded	overcrowded	too crowded
over-	too much	flow	overflow	flow over the edge
under-	too little	cook	undercook	cook too little
under-	too little	estimate	underestimate	guess too low
under-	below/beneath	ground	underground	below the ground
under-	too little	age	underage	below the required age

Key Rules

Use im- before m and p: If the base word starts with m or p, use im-. Think of "impossible" and "immature". The doubled letter (mm, pp) makes it easier to say.
Use ir- before r: If the base word starts with r, use ir-. Think of "irregular" and "irresponsible". The doubled letter (rr) is the clue.
Use il- before l: If the base word starts with l, use il-. Think of "illegal" and "illogical". Again, the doubled letter (ll) is the clue.
Use in- for most other words: If the base word does not start with m, p, r, or l, use in-. Think of "incorrect" and "incomplete".
Over- and under- are opposites: "Overcook" means too much cooking, and "undercook" means too little. Many base words can take both prefixes.
The base word does not change spelling: When you add a prefix, do not change the spelling of the base word. "il- + legal = illegal" (double l is correct). "im- + polite = impolite" (not "ipolite").

Common Mistakes

Wrong	Right	Why
The answer is inpossible.	The answer is impossible.	"Possible" starts with p, so use im-, not in-
His behaviour was inresponsible.	His behaviour was irresponsible.	"Responsible" starts with r, so use ir-, not in-
That move was inlegal.	That move was illegal.	"Legal" starts with l, so use il-, not in-
Do not over-cook the rice.	Do not overcook the rice.	Over- and under- are usually written as one word, without a hyphen
She was being very umpatient.	She was being very impatient.	"Patient" starts with p, so use im-; there is no "um-" prefix
The handwriting was unleadable.	The handwriting was illegible.	The correct word is "illegible" (il- + legible), not "unleadable"

Clue Words

Words that signal negative prefixes (in-, im-, ir-, il-)

not, unable to, opposite of, without, lack of

Words that signal over-

too much, more than needed, excessively

Words that signal under-

too little, not enough, below, less than needed

Common base words for these prefixes

possible, patient, polite, mature, regular, responsible, legal, legible, logical, correct, complete, visible, cook, sleep, crowd, estimate

Tip: Look at the first letter of the base word to pick the right negative prefix. If it starts with m or p, use im-. If it starts with r, use ir-. If it starts with l, use il-. For everything else, use in-. Remember: the prefix letter matches the first letter of the base word!

Practice Tips

The letter-match trick: When you see a word that means "not [something]", check the first letter of the base word. Match it to the correct prefix using the rule: m/p = im-, r = ir-, l = il-, others = in-.
Over/under pairs: Make a list of base words that can take both over- and under- (cook, estimate, charge, pay). Write sentences using each pair to see how the meaning flips.
Strip the prefix: When you see a long word you do not recognise, try removing the prefix to find the base word. "Irresponsible" becomes "responsible" when you remove "ir-". This helps you work out the meaning.
Spelling check: After adding a prefix, read the word aloud. If it sounds awkward (like "inpossible"), you probably chose the wrong prefix. The correct form should flow smoothly ("impossible").

Quick Reference

Choosing the Right Negative Prefix

First Letter of Base Word	Prefix	Example
m or p	im-	im + possible = impossible
r	ir-	ir + regular = irregular
l	il-	il + legal = illegal
other letters	in-	in + correct = incorrect

Over- and Under- at a Glance

Prefix	Meaning	Examples
over-	too much	overcook, oversleep, overcrowded, overflow
under-	too little	undercook, underestimate, underage, underground

Prefixes (P4) (Primary 4)