Meaning
Can and could always convey potentiality. In this sub-flavor of Praxic modal verbs, potentiality equates to what the subject is capable.
- That capability may be due to their innate talents or developed skills, whether physical, mental, social, or otherwise (Ability).
- Or it may be due to resources at their disposal, including tools, time in their schedule, information required to complete the task, financial flexibility, etc. (Capacity).
The proximal modal can is used for present capabilities and for situations that are not time-specific. Its distal counterpart could is used for past situations, as well as unreal situations (which includes most future applications).
The periphrastic expression be able to also expresses capability.
Structure
Modal Auxiliaries
Can and could have the same properties as any other modal verb:
- They should only ever be in the Bare Infinitive form (the base form); we never say “I am
couldinglift it”, or “Shecansjuggle”. Since they cannot take any other form, they are called defective. Example: “could lift 200lbs”, or “can juggle” - The form of the verb that follows can/could — whether that is the main verb or another auxiliary verb — should also be in the Bare Infinitive form (the base form). Example: “can dance“, or “couldn’t believe“
- Modal verbs should always be at the front of the verb string, before any standard auxiliary verbs (BE, HAVE, or DO).
- Modals behave like other auxiliary verbs, particularly as leading auxiliaries:
- Negators like not (sometimes in its contracted form of n’t) or never come immediately after the auxiliary. Example: “can never open”, or “couldn’t write”
- The auxiliary can come before the subject (called ‘subject-auxiliary inversion’) for situations like some question forms. Example: “Could he have done it?”, or “Can‘t you move any faster?”
- The auxiliary can represent the full verb string (called ‘ellipses’) in structures like short answers. Example: “Yes, I can.” or “No, they couldn’t.”
Can is a special word in that its negative form has three variations: the basic “can not“, the contracted “can’t“, and the unique compound “cannot“. The three are interchangeable and are selected based on preference or on emphasis.
For more details on the forms of modals, visit the Modal Verbs page.
Perfect Modals
Could can also be in its Perfect Modal form could have for unreal past situations, as in “She could’ve won first prize had she practiced more consistently.” The have should always be in the form of ‘have’ (never ‘has’ or ‘had’) or its contracted form. The following verb should be in its Perfect Participle form (the -ed/en form).
Be Able To
The phrase be able to also indicates capability and can act as a substitute for can/could. Instead of the properties of a modal verb, it has the same properties as normal verbs:
- Be can change forms based on tense or based on different speakers, like “is able to“, “are able to“, “was able to“, “has been able to“, etc. In other words, this phrase is not defective.
- The form of the verb that follows should be in an infinitive form. Together with the to, they may be regarded as Full Infinitives. The resulting verb constructions are similar to Verb Patterns. For example: “am able to grasp” or “were able to see“.
- The full phrase be able to does not behave like an auxiliary verb, but that initial BE does. For instance, we might say, “Were you able to find your contacts?” – “No, I wasn’t.” Notice how the negator not comes after ‘was’, or whatever for BE takes.
Since can and could are defective, they cannot be used in some situations and may be grammatically awkward in others. The phrase be able to can then be a substitute, since it is more grammatically flexible.
- A common example is double modals. A sentence like “We might could do it,” is considered poor grammar in most English dialects (although for some less common dialects, it is perfectly acceptable). So instead, one might say “We might be able to do it.”
- Another example is when Modals of Capability apply to the second part of Verb Patterns. The sentence “She started
can walkingwhen she turned 11 months,” is incorrect (and “She startedcanning walk…” is even worse). So instead, one would say “She started being able to walk when she turned 11 months.”
Be able to is the best periphrastic substitute for ability and capacity, and it works for nearly every application. Weaker substitutes (ones that are more selective) include but are not limited to: be equipped to, be fit to, be prepared to, and be qualified to.
Usage
Skill, Aptitude, and Capacity
Abilities usually refer to the characteristics of the subject (specifically, their skill or aptitude). Unlike Epistemic or Deontic Modals, Praxic Modals like those of ability or habit are generally not based on the speaker’s understanding, but rather on the subject’s performance.
In most (but not all) situations, the affirmative applications of can, could, and be able are used because the subject has demonstrated their ability to carry out that action before, perhaps repeatedly, or else has demonstrated a comparable action. But on occasion, we might use can/could out of pure belief, like the inspirational “You can do it!”
In some situations, the ability has less to do with the subject’s innate abilities and more to do with their capacity according to the situation at the time. Consider the following:
- Mitchell says, “Yes, I can lift that,” regarding a heavy moving box. This reflects his physical ability.
- Mitchell says, “Sorry, I can’t lift that right now,” regarding that same box. In this situation, he hasn’t gotten any weaker, but his present situation provides some hindrance to the possibility of him performing that action. Perhaps his arms are already full with other boxes.
The distinction between these meanings is not evident through grammar, but is usually understood through context. Sometimes, however, the meaning is ambiguous.
Capability with Tense and Mood
Capability with Tense
Both for present tenses and for situations that are not time-specific, we use can for ability. eg: “I can open that for you,” (right now) or “She can help you with your homework,” (whenever).
For past abilities, we use could. eg: “Dave could juggle when he was eight,” (noting when he started his hobby) or “Jean could eat an entire pizza in one sitting,” (but not anymore).
Capability with Mood
Could may be used for the Unreal Mood. Usually, the capability itself (or the lack thereof) is real, but it is the performance of the verb that is unreal.
For the unreal past, we use the Perfect Modal could have. eg: “You could’ve spoken up,” (the subject was capable, but didn’t speak up) or “I couldn’t have done any better if I had tried,” (but the subject didn’t try).
In its base form, could can also be applied to hypothetical situations like “Could you jump over that gap?” By extension, this applies to most future forms — plans or predictions that are unreal scenarios, at least until the become realized (or not) when that time eventually becomes the present. eg: “She could become president when she grows up!” (On the other hand, there are some real future forms such as appointments, for which we use can, as in “You can return tomorrow morning when the doors open at 7:00.”)
Advanced Concepts
Perception & Cognition
On occasion, we use can/could not just to express a capability, but also to express that the verb is real and is taking place in the moment (where it may not have taken place previously). This typically applies to verbs of perception or cognition.
For example, upon reaching the peak of a small mountain, a hiker might say, “I can see my house from here!” That statement not only conveys the hiker’s ability or capacity to see their house from that height, but also the fact that the hiker does see their house. The sentence “I see my house from here,” is just as grammatically appropriate, but most native English speakers are more likely to use can in situations like this, probably because they specifically want to communicate the (perhaps surprising) capability more than they want to communicate their action.
Relation to Other Modals
Other Modals in the Praxic Flavor
Each of the core modal verbs has multiple meanings, and sets of meanings are set into modal flavors. Modals of Capability are a subset of Praxic Modals, which express what the subject sometimes does (even if they are not doing it at the moment). Another subset of this flavor is Modals of Habit & Inclination, as in “Kids will be kids,” and “She used to play piano every day when she was younger,”. We use the modals will, would, and might, the pseudo-modals used to and tend to, and other expressions for Modals of Habit & Inclination.
Other Flavors of Can/Could
When considering can/could across different flavors, they convey modals of capability, permission, possibility, and willingness; these are all different types of potentiality. In some instances, the meaning of can/could blurs the line between flavors. For example, “Susan can drive you home,” could reflect both Susan’s capacity and her willingness, or it could be offered as a possibility when brainstorming ways to get home. It could even reflect permission if there was some restriction in place for whatever reason. In any case, the listener understands that taking a ride with Susan is an option.
Teaching Tip
Setting a Precedent
Can for ability is usually the first modal that non-native English speakers learn. Once students have become familiar with this and Modals of Capability are revisited, focus on what makes modal verbs unique, especially regarding their form (for instance, show how they are defective, even though you might not use that term as you should grade the explanation based on the students’ proficiency level).
Later, when students encounter may, might, should, and the rest, show how these modals play by the same special rules as can, which learners should be comfortable with by then.
Further Reading
Encyclopedia Pages
- Habits ← more modals in the Praxic flavor
- Modals of Permission, Advice, & Obligation ← can/could in a different modal flavor
- Modals of Possibility & Probability ← can/could in a different modal flavor
RUDIMENTARY Topics
CEFR Levels A1-A2: Beginner/Starter to Pre-Intermediate courses
