How to use “Could,” “Would,” and “Should”: Grammar rules and differences explained

When learning English, there is often confusion around the modal verbs: “could”, “should”, and “would”. To understand their differences, let’s answer the question of what modal verbs are and how they differ from regular verbs.

Modal verbs in English are a special type of verb that are used together with a main verb to express possibility, intention, ability, or necessity. They differ from regular verbs in that they cannot be used independently and always require an additional verb in the infinitive without “to”. For example, in the sentence “I can swim”, “can” is the modal verb indicating the person’s ability to swim, and “swim” is the main verb. It’s important to understand a few key features of modal verbs:

• They don’t change form based on tense, person, or number. For example, “he can”, “they can”.
• They don’t require adding “do/does” in questions or “don’t/doesn’t” in negations.
• They always precede the main verb in the infinitive.

Modal verbs include words like “can”, “could”, “may”, “might”, “shall”, “should”, “will”, “would”, “must”. Now that we understand the basic concepts, let’s examine the difference between “could”, “should”, and “would” in English. In brief, could is used to express possibility. For example, “I could go to the store” means that I have the possibility to go to the store. Should is used to give advice or a recommendation. For example, “You should study for the test” means you ought to study for the test. Would is used to express a hypothetical situation or for polite offers/requests. For example, “I would travel if I had time” means I would travel if I had the time.

Let’s examine the modal verbs “could”, “should”, and “would” in more detail, including rules for usage, grammar, and more examples.

Could – Meaning, Usage Rules, and Examples

Could is typically used to express possibility or permission in the past tense, as well as to indicate hypothetical possibilities in the future.

The Form of “Could”:

• Could comes at the start of the verb phrase (after the subject and before the other verb): “We could have lunch early.”
• Could cannot be used with another modal verb: “We could drive to France,” but not “We could might drive to France.”

Using “Could”:

• Possibility: “Could” is often used to express possibility in the present and future tense. For example, “It could be blue.”
• Suggestions: “Could” is also commonly used for suggestions: “I could go as Julius Caesar.”
• Permission: “Could” is used to ask for permission. “Could” is more formal and polite than “can”: “Could I ask you a personal question?”

“Could” in the Past Tense:

• Abilities: “Could” is used to talk about abilities in the past: “When I was young, I could easily touch my toes.”
• Possibility: “Could have + past participle” is used to talk about possibility in the past: “I could have been a lawyer.”

Could have + past participle: Could is also used with have + past participle to express a missed opportunity in the past:

• “I could have studied harder.”
• “They could have arrived earlier if they left on time.”

Use could and couldn’t for ability in the past (they are the past forms of can and can’t):

• When I was younger, I could run a mile in 7 minutes. Now it takes me 20 minutes!
• Yesterday, I couldn’t find my wallet anywhere – but this morning I found it.
• Last year, he couldn’t speak English very well, but now he can.

Use could to talk about future possibilities:

• “Do you have any ideas for our publicity campaign?”
• “Yes, I’ve got a few ideas. I could put advertisements on Facebook and Google.”
• “We could also give out pamphlets in our neighborhood. Maybe John could even contact local TV stations.”

Use could to make polite requests:

• Could you please open the window? It’s hot in here.
• Could you turn the music down? Thanks.
• Could you make 10 copies of this report, please?

Questions and Negatives with “Could”:

• To form questions, the subject and “could” switch positions, and no “do/does/did” is used: “Could I pay by credit card?”

Should – Meaning, Usage Rules, and Examples

“Should” is used to express recommendations, obligations, or advice.

The Form of “Should”:

• Should is used at the start of the verb phrase (after the subject and before the other verb): “You should check the weather.”
• Should cannot be used with another modal verb: “You should try to relax,” but not “You should must try to relax.”

Using “Should”:

• Recommendations and Advice: “Should” is often used for giving advice or recommendations. For example, “You should see a doctor.”
• Obligation or Necessity: “Should” is also used to indicate an obligation or necessity, often when criticizing someone else’s actions: “You should be wearing a seatbelt.”
• Probability: “Should” is used to indicate that something is likely or expected: “The train should arrive soon.”

Should for obligation/necessity vs recommendations/advice: When should is used to express obligation or necessity, it has a more insistent, definitive tone:

• “You should wear a seatbelt.”
• “They should follow the rules.”

But to express advice or a recommendation, should has a more suggestive tone:

• “You should try this cake, it’s delicious.”
• “She should get more sleep.”

Use should and shouldn’t to ask for and give advice and suggestions:

• “I’ve had a really bad headache for the past week.”
• “That’s not good – you should go to the doctor.”
• “I want to make more friends, but I don’t know how.”
• “First of all, you shouldn’t spend so much time on the computer. You should go out and join a club or start playing a sport instead!”
• “I had a fight with my best friend. What should I do?”
• “Hmm… I think you should call her and tell her you’re sorry.”

“Should” in the Past Tense:

• Should have + past participle: Used to talk about things that would have been ideal in the past but didn’t happen. This can express regret: “I should have studied more.”

Questions and Negatives with “Should”:

• To form questions, the subject and “should” switch positions, and no “do/does/did” is used: “Should we call a taxi?”

Would – Meaning, Usage Rules, and Examples

“Would” is a complex modal verb that, depending on the context, can indicate desire, hypothetical situations, or polite offers and requests.

The Form of “Would”:

• Would is used at the start of the verb phrase (after the subject and before the other verb): “She would write a letter.”
• Would cannot be used with another modal verb: “He would go to the cinema,” but not “He would can go to the cinema.”

Using “Would”:

• Past Habits: “Would” is often used to describe past habits. For example, “When I was a child, I would play outside until dark.”
• Conditionals: “Would” is used in conditional sentences to express hypothetical situations. For example, “If I had a million dollars, I would travel the world.”
• Polite Offers and Requests: “Would” is used for polite offers or requests. For example, “Would you mind closing the window?”

“Would” in the Past Tense:

• Would have + past participle: Used to talk about actions that could have happened in the past but didn’t. For example, “I would have called you, but I didn’t have your number.”

Questions and Negatives with “Would”:

• To form questions, the subject and “would” switch positions, and no “do/does/did” is used: “Would you like some tea?”

Use would to talk about unreal or unlikely situations:

• If I were the president of my company, I would make a lot of changes.
• If people were more generous, there wouldn’t be so much poverty in the world.
• She would travel around the world if she had more vacation time.

In this case, would is often shortened to ‘d

• If I were the president of my company, I’d make a lot of changes.

Use Would you like…? to make polite offers:

• “Would you like anything to drink?”
• “A soda would be great. Thanks!”
• “Would you like to join us for dinner?”
• “I’d love to, but I actually have other plans tonight.”
• “Would you like to see some pictures from my vacation?” “Sure!”

Don’t use “to” after should, could, or would:

• You shouldn’t to smoke. (Incorrect)
• You shouldn’t smoke. (Correct)
• We could to order pizza tonight. (Incorrect)
• We could order pizza tonight. (Correct)
• I would to buy a new car if I had the money. (Incorrect)
• I would buy a new car if I had the money. (Correct)

Could expresses theoretical possibility in the present/future or ability in the past. Should indicates a recommendation, advice or obligation. Would expresses a hypothetical/desired situation or a polite offer/request. Remember these key distinctions, and you’ll be able to properly use these modal verbs in English.