There are three common English verbs that everyone knows but many misuse: could, should, and would. All three verbs are in the past tense. All three are modal verbs. And all three, being past tense, are commonly paired with the auxiliary verb have – usually as a contraction. But they will mess you up.

• Could is the past tense of the verb can.

“Can you do this?” “I could if I had the time.”

• Should is the past tense of shall.

“I shall get to work on this right away.” “I should have waited until I had all the instructions.”

• Would is the past tense of will.

“Will you do this for me?” “I would have been happy to do that for you.”

They also have their roots in Old English. Could was cunnan, the Old English word for know, or certainty. Should was sceolde, the past tense of shall be. And would was wolde, the past tense of wyllan, which became the modern English word will.

Modal Verbs

Modal verbs express a conditional meaning to the main verb. They are used to connote:

Possibility

• You could call me tonight if you get the chance.

Permission

• Should I leave?

Speculation

• Would you happen to know what the password is?

Politeness

• Could you close the door on your way out?

Prohibition (with a negative)

• You shouldn’t ask a person about their age.

The problem that learners and even native speakers often encounter, is a failure to recognize the difference between the three words. Consider the sentence, “You could call me tonight if you get the chance.” Substituting should for could turns the sentence into a command: “You should call me tonight if you get the chance.” If you stop to think of the present tense of the verb – can, shall, or would – you are more likely to avoid this mistake.

The Problem with Have

When it follows the modal verb, the main verb must take the past participle form.

• You could have warned me that your sister was coming to visit.
• You should have closed the windows if you knew that it was going to rain.
• I would have liked to have seen that movie last night.

When used together, these modal verbs and this auxiliary verb are often formed into conjunctions – could’ve, should’ve, and would’ve. And here we arrive at not one, but two of the most common mistakes made in English.

Typically, when we speak English, we fail to enunciate clearly. The difference between could’ve and could of may seem obvious when written, but not in verbal communication. Therefore, many English speakers, including many native speakers, believe that the phrases could of, should of, and would of are proper. Let me assure you, although I see and hear this almost every day, they are not!

This also happens when the negative form of the modal verb is used.

• I couldn’t have been prouder of my niece than I was last night at the awards ceremony.

Not

• I couldn’t of been prouder of my niece than I was last night at the awards ceremony.

Then, as if that wasn’t enough of a problem, certain areas of the United States speak so quickly, and with such a strong accent, that the result is a common mispronunciation of the conjunctions as coulda, shoulda, woulda. This has now become a famous idiom that expresses regret. “I could have done it, I should have done it, and if I knew then what I know now, I would have done it.” Or, abbreviated, we simply say, “Coulda, shoulda, woulda.”