Fourth Grade English: Modal Auxiliaries (Can, May, Must, Should)

Language is not limited to stating what definitely happens; it also allows us to express conditions, possibilities, permissions, abilities, and moral duties. In English, we convey these shades of meaning using a special family of helping verbs called modal auxiliaries (or modal verbs). By mastering words like can, may, must, should, could, might, and would, you will gain the power to convey precise attitudes and subtle degrees of certainty in both speaking and writing.

What Are Modal Auxiliaries?

A modal auxiliary is a helping verb that pairs with a main verb to express ability, permission, obligation, possibility, or advice.

+-----------------------------------------------------------------------+
|                    THE MODAL AUXILIARY SPECTRUM                       |
+-----------------------------------------------------------------------+
|                                                                       |
|   ABILITY      PERMISSION     POSSIBILITY    ADVICE      NECESSITY    |
|   "can"        "may"          "might"        "should"    "must"       |
|                                                                       |
|   "I CAN swim" "MAY I go?"    "It MIGHT rain""You SHOULD""We MUST     |
|                                               study"      wear seat-  |
|                                                           belts"      |
+-----------------------------------------------------------------------+

Unique Characteristics of Modal Auxiliaries

Modal auxiliaries differ from all other verbs in three significant ways:
1. No "-s" Ending: Modal verbs never add "-s" in the third person. We say "He can run," never "He cans run."

2. Followed by Base Form: A modal verb is always followed directly by the base form of the main verb without "to" (can jump, must leave, should listen).

3. No "-ing" or "-ed" Forms: Modal verbs cannot be turned into participles. There is no such word as "canning" or "musted."


Core Modal Verbs and Their Meanings

Each modal auxiliary communicates a specific intention or condition.

+-----------------------------------------------------------------+
|                  MODAL AUXILIARY MEANINGS & USAGE               |
+-----------------------------------------------------------------+
| Modal Verb | Primary Meaning          | Mentor Sentence         |
+------------+--------------------------+-------------------------+
| Can        | Physical/Mental Ability  | Elena can solve the     |
|            |                          | complex puzzle.         |
| May        | Formal Permission or     | You may borrow my pen.  |
|            | Possibility              | It may snow tonight.    |
| Must       | Strong Obligation or     | Drivers must stop at    |
|            | Logical Certainty        | red lights.             |
| Should     | Recommendation or Advice | You should eat healthy. |
| Could      | Past Ability or Possibility| He could run fast.   |
| Might      | Weak Possibility         | We might visit grandmas.|
| Would      | Conditional or Habit     | I would travel if rich. |
+-----------------------------------------------------------------+

Can vs. May: Ability vs. Permission

The classic distinction between "can" and "may" is an essential rule of polite and precise speech:
- "Can" refers to physical power, skill, or capacity: "Can you touch the top shelf?" (Do you have the height or ability?)

- "May" asks for or grants formal permission: "May I be excused from the table?" (Do I have your permission?)


Must vs. Should: Requirement vs. Recommendation

Understanding the difference in intensity between "must" and "should" helps you set the appropriate tone in persuasive writing:
- "Must" indicates a mandatory requirement or absolute law: "All passengers must fasten their seatbelts."

- "Should" offers helpful guidance, moral duty, or advice: "You should review your notes before the quiz."


Modals Expressing Possibility and Degrees of Certainty

Modal verbs allow writers to calibrate how certain they feel about a statement.

+-------------------------------------------------------------+
|                DEGREES OF CERTAINTY CONTINUUM               |
+-------------------------------------------------------------+
|  100% Certain:  "The eclipse WILL occur tomorrow."          |
|   90% Certain:  "The team MUST be exhausted after 5 miles." |
|   50% Certain:  "It MAY rain this afternoon."               |
|   30% Certain:  "We MIGHT see a shooting star tonight."     |
+-------------------------------------------------------------+

Nuance in Informative Writing

In scientific and informational texts, researchers rarely speak in absolute certainties about unproven theories. Instead, they use modal auxiliaries like "may," "might," and "could" to present cautious, well-grounded hypotheses: "Rising temperatures might alter migration patterns."

Practice Exercises

Practice Questions

  1. Choose the modal verb that expresses formal permission: "Students (can / may) begin the test once everyone is seated."
  2. Choose the modal verb that indicates a strict requirement: "Citizens (should / must) obey traffic signals."
  3. Identify the modal auxiliary in this sentence and state its function: "You should stretch your muscles before running the sprint."
  4. Fill in the blank with "can" or "may" to ask for permission: "______ I please borrow your pencil sharpener?"
  5. Rewrite this sentence using a modal verb to express possibility rather than certainty: "The spaceship lands on Mars today."

Answer Key and Explanations

  1. Correct word: "may" (used for formal permission).
  2. Correct word: "must" (indicates mandatory legal obligation).
  3. Modal auxiliary: "should." Function: giving advice or making a recommendation.
  4. "May" (asks for polite permission).
  5. "The spaceship may (or might / could) land on Mars today." (Using "may" shifts the statement from certainty to possibility).