The Grammar of Certainty: Mood vs. Modality

Ever find yourself hesitating between “He might be late” and “He could be late,” wondering if there’s a secret rule you missed in school? You’re not alone. This subtle distinction is a gateway to a fascinating area of grammar: how we express certainty, possibility, and obligation.

Language isn’t just for stating cold, hard facts. It’s a flexible tool for conveying our beliefs, doubts, wishes, and commands. English accomplishes this through two parallel systems that often get confused: grammatical mood and lexical modality. They might sound like technical jargon, but understanding them unlocks a deeper appreciation for the nuance in our everyday speech. Let’s break it down.

What is Grammatical Mood? The Verb’s “Attitude”

At its core, mood (or mode) is a grammatical feature of a verb. It’s not about adding extra words; it’s about changing the verb’s form or structure to signal the speaker’s attitude toward what they’re saying. Think of it as the verb dressing up for a different occasion—a statement of fact, a command, or a hypothetical situation.

In modern English, we primarily use three moods:

1. The Indicative Mood

This is your default setting. The indicative mood is used to state facts, express opinions, and ask questions. 99% of the sentences you speak or write will be in the indicative mood. It deals with the world as we perceive it to be.

• She is a doctor. (Fact)
• I think that movie was too long. (Opinion)
• Did you finish the report? (Question)

2. The Imperative Mood

The imperative mood is for giving commands, instructions, or making requests. It’s direct and to the point. The verb is used in its base form, and the subject (usually “you”) is implied.

• Close the door.
• Be quiet.
• Please pass the salt.

3. The Subjunctive Mood: The Ghost in the Machine

Here’s where things get interesting. The subjunctive mood is the mood of unreality. It expresses things that are not true, hypothetical, wished for, or demanded. It was once a powerhouse in English, but its use has dwindled, making it feel formal or even a bit archaic today. However, it’s still alive and kicking in certain structures.

You’ll recognize it by its distinct verb forms, most famously using “were” instead of “was” and the base form of the verb where you’d expect an “-s”.

• Wishes/Hypotheticals: “If I were you, I wouldn’t do that”. (I am not you, so this is contrary to fact.)
• Wishes/Hypotheticals: “I wish it were Friday”. (It is not Friday.)
• Demands/Suggestions: “The committee insists that he be present at the meeting”. (Not “he is” or “he should be”.)
• Demands/Suggestions: “I recommend that she submit her application early”. (Not “she submits”.)

The subjunctive is a structural, grammatical way to signal that we’ve left the realm of reality. But English has another, more common way to do the same thing.

Enter Modality: The Power of Helper Verbs

If mood changes the verb itself, modality adds a new word to the mix. Modality is the system of using helper verbs—called modal verbs—to express degrees of certainty, possibility, ability, permission, or obligation.

This is where our day-to-day expressions of nuance come from. The main modal verbs are:

can, could, may, might, must, shall, should, will, would

Linguists often sort these into different types of modality:

Epistemic Modality: The Grammar of Belief

This is about how certain we are about something. It relates to knowledge and possibility.

• He left an hour ago. He must be home by now. (High certainty)
• She should be in her office. (Strong expectation)
• It may rain later. (Possibility)
• They might not agree with the plan. (Possibility, perhaps a bit weaker than “may”)

Deontic Modality: The Grammar of Rules and Duty

This type of modality deals with obligation, permission, and prohibition.

• You must wear a helmet. (Strict obligation)
• You should apologize. (Recommendation/moral obligation)
• You may begin the exam. (Permission)
• You can borrow my car. (Informal permission)

The Showdown: Mood vs. Modality

So, what’s the real difference? Think of it this way:

• Mood is grammatical. It’s baked into the verb’s form (e.g., is vs. be vs. were). It’s a closed, structural system.
• Modality is lexical. It uses separate words (modal verbs) to add meaning. It’s a flexible, nuanced system that has largely taken over the duties of the old subjunctive mood.

Consider this classic subjunctive sentence:

“I suggest that he go“. (Subjunctive Mood)

In modern, everyday English, you’re far more likely to hear it expressed using a modal verb:

“I suggest that he should go“. (Lexical Modality)

Both are correct, but the second one uses the modal system that we are now more comfortable with. Modality gives us a whole palette of colors to paint with, whereas mood gives us a few structural choices.

So, What About “Might Be Late” vs. “Could Be Late”?

We can finally answer our opening question. “He might be late” and “He could be late” are both examples of epistemic modality. They both express possibility.

In most contexts, they are virtually interchangeable. The difference in meaning is incredibly slight and often depends more on the speaker and context than on a hard-and-fast rule.

• “He might be late” and “He could be late” both convey that there is a possibility of his lateness.

If we are forced to find a difference, some linguists and style guides suggest a subtle one:

• Could sometimes feels a little more grounded in specific circumstances. “With all this traffic, he could be late”. It’s a possibility that has been enabled by the situation.
• Might can sometimes feel a bit more purely speculative, or perhaps imply a slightly lower chance than may.

But be careful! Could also serves as the past tense of can (“When I was young, I could run very fast”), a function might doesn’t have. Context is everything.

The takeaway? For expressing future possibility, don’t lose sleep over might vs. could. They are two different tools in the modality toolbox that do a very similar job.

Why This Matters

Understanding the difference between mood and modality is more than just a grammar lesson. It reveals the elegant way language evolves. While the grammatical subjunctive mood has faded, our need to express uncertainty hasn’t. Instead, we’ve developed a rich and flexible system of modal verbs to do the heavy lifting.

By mastering both, you become a more precise communicator—knowing when to use the formal, structural power of the subjunctive (“If I were king…”) and when to use the subtle, everyday nuance of modality (“Well, I guess he might be king… one day”).