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Have you ever stopped mid-sentence, trying to figure out how to put the verb “can” into the future tense? You can say “I will go” and “I will eat”, but “I will can” sounds instantly, fundamentally wrong. The same goes for saying “I am musting” or trying to use “ought” in the past tense. It feels like hitting a grammatical brick wall.

This isn’t a failure on your part as a speaker. You’ve stumbled upon one of linguistics’ most curious corners: the world of defective verbs. These aren’t verbs that are “broken” or “bad.” Rather, they are verbs with gaps in their playbook—they’re missing some of the standard forms that regular verbs have in their toolkit.

What Makes a Verb ‘Defective’?

Think of a typical verb like walk. You can conjugate it in almost any way imaginable:

• Infinitive: to walk
• Present tense: walk / walks
• Past tense: walked
• Present participle: walking
• Past participle: walked

Now, try to do the same with a verb like must.

• Infinitive: to must (Nope)
• Present tense: must (Okay, but no “he musts”)
• Past tense: musted (Doesn’t work)
• Present participle: musting (Definitely not)

As you can see, must is missing most of its principal parts. It’s “defective” because its conjugation pattern is incomplete. In English, the most famous group of defective verbs are the ones we use every single day: the modal auxiliary verbs.

The Usual Suspects: English Modal Verbs

The core group of English modal verbs includes can, could, may, might, shall, should, will, would, and must. They are grammatical workhorses, used to express modality—concepts like ability, permission, possibility, and necessity. They’re also all defective, which is why they share a few strange characteristics:

1. They don’t have infinitives or participles. This is why you can’t say “to can”, “maying”, or have “musted.” They only exist in a simple present or past tense form.
2. They don’t take an “-s” in the third-person singular. We say “she walks”, but never “she cans” or “he wills.”
3. They are always followed by a bare infinitive (the infinitive form of a verb without “to”). You say “I must go“, not “I must to go.”

A Fossil in Our Language

So why are they like this? The answer lies buried in the history of the English language. These verbs are linguistic fossils, remnants of an ancient Germanic verb class called the preterite-present verbs.

In Old English, the present tense forms of these verbs (like can or shall) already looked like the past tense (or ‘preterite’) forms of other strong verbs. For instance, the original meaning of can was closer to “I have learned” or “I know.” The learning part happened in the past, but the result—the knowledge or ability—exists in the present. Because their present tense was already “borrowed” from a past-tense structure, they couldn’t form a normal past tense. Instead, they had to create new, distinct past forms (like can → could).

This complicated history left them without the building blocks needed to create infinitives or participles. They were never “full” verbs to begin with, and they’ve carried that legacy into Modern English.

Filling the Gaps: The Genius of Suppletion

Language, however, despises a vacuum. If a verb can’t do a certain job, speakers will find a workaround. When we need to express the meaning of a modal verb in a tense it doesn’t have, we perform a clever substitution. This is known in linguistics as suppletion, where gaps in one word’s forms are filled by forms from a completely different word.

Consider our original problem: how do you put “can” in the future tense? You can’t say *”I will can.”* Instead, you swap in a phrase that means the same thing:

• “I will be able to attend the meeting.”

Here, “be able to” acts as a stand-in for the missing future tense of “can.” We do this all the time without thinking:

• For the past tense of “must”: We don’t say *”Yesterday, I musted finish my report.”* We say, “Yesterday, I had to finish my report.”
• For the present participle of “must”: “I’m having to work on a Saturday” fills the role of the non-existent “musting.”

This suppletive strategy is a perfect example of how languages evolve pragmatically. If the built-in grammar has a hole, speakers simply patch it with another tool.

Grammatical Black Holes Around the World

This phenomenon isn’t unique to English. Many languages have verbs with curious gaps in their conjugations.

• Russian: The verb победить (pobedit’), meaning “to win” or “to be victorious”, has a famously defective form. There is no standard way to say “I will win” in the first-person singular future tense. *”Я победю”* (Ya pobedyu) sounds childish or incorrect. Instead, speakers use a workaround, like saying “Я одержу победу” (Ya oderzhu pobedu), which means “I will gain a victory”, or they rephrase the sentence entirely.
• Latin: The classical language had several defective verbs. The verb aio (“I say”) only existed in a few forms, and memini (“I remember”) was another preterite-present verb that existed only in the perfect tense but was used with a present meaning.
• Irish: The Irish language has two verbs for “to be.” The verb is (the copula) is used for identity and classification (“He is a doctor”). It’s highly defective and only exists in a few tenses. For all other situations (like location or temporary states), you must use the verb tá. You can’t just pick one; the grammar forces you to use the right verb for the right job, as is has too many gaps to be a general-purpose verb.

Embracing the Imperfections

Defective verbs are more than just a quirky exception for language learners to memorize. They are a window into the messy, beautiful, and ever-changing nature of human language. They show us that grammar isn’t always a neat and tidy system designed by a committee; it’s a living artifact, shaped by centuries of use, reuse, and clever improvisation.

So the next time you find yourself unable to say “musting” or “to can”, don’t feel frustrated. Smile, and appreciate that you’ve just bumped into a fascinating piece of linguistic history, a grammatical gap that tells a story thousands of years old.

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