Three small words—could, should, and would—trip up even advanced English learners. They all look similar, and all three appear in conditional sentences. So how do you know which one to use?
The short answer: each word expresses a different kind of meaning. Could expresses possibility. Should expresses opinion or high likelihood. Would expresses a definite (but hypothetical) outcome. In this post, you’ll get a quick review of the four conditionals, a clear explanation of each word with examples, and a reference table you can come back to whenever you need a reminder.
Table of Contents
A Quick Review of the Four Conditionals
Conditional sentences express cause and effect. English has four types, each describing a different kind of situation.
The zero conditional describes cause and effect that is always true. “When the temperature drops below 0°C, water freezes.” There’s no uncertainty here—it always happens.
The first conditional describes a realistic prediction about the future. “If it’s sunny tomorrow, we’ll go to the beach.” The speaker thinks this is likely to happen.
The second conditional describes a hypothetical or unlikely situation in the present or future. “If I magically grew wings, I would be able to fly.” It’s theoretically possible, but not expected.
The third conditional describes a past situation that didn’t happen—and its imagined result. “If Columbus had not sailed to America, someone else eventually would have.” The past can’t be changed, so this is purely hypothetical.
Note: For a deeper look at each conditional type, see the full conditionals overview and the dedicated third conditional guide on this blog.
Using “Could” in Conditional Sentences
Could expresses possibility. When something could happen, it means it is one of several possible outcomes—not certain, not required, just possible. It doesn’t express desire or recommendation. It simply states what is possible, or what was possible in the past (even if it didn’t happen).
Examples
First conditional: If it rains enough tomorrow, it could flood. We should be prepared.
Second conditional: If we moved to a bigger apartment, we could set up a home office, or we could convert the extra room into a gym.
Third conditional: If I had known you were free yesterday, I could have asked you to join me for coffee.
Pro tip: Notice that could often appears in the second conditional alongside other options (“we could X, or we could Y”). That’s because could highlights that multiple outcomes are possible—not just one fixed result.
Using “Should” in Conditional Sentences
Should has two uses in conditional sentences. First, it expresses an opinion about the best course of action—a recommendation. Second, it expresses that something is very likely to happen (or was very likely to have happened in the past).
Examples
First conditional (high likelihood): If it continues to rain this hard for another hour, it should start flooding. You should start preparing now.
Third conditional (suggestion / past likelihood): If you were going to the game yesterday, you should have invited me. You should have known I had the day off because I sent you an email.
Note: Should doesn’t appear in the second conditional. The second conditional describes unlikely hypothetical situations, and it rarely makes sense to give advice about something that almost certainly won’t happen.
Using “Would” in Conditional Sentences
Would describes what will definitely happen if (and only if) specific circumstances change. It’s used for hypothetical but certain outcomes—”if X were true, Y would happen.” It can also describe a past event that didn’t occur because the right circumstances weren’t there.
Examples
Second conditional: If you studied abroad for a year, your English would improve dramatically. Given that condition, the outcome is certain.
Third conditional: If I had known you had the day off yesterday, I would have asked you to join me. I wanted you there, but I didn’t know you were free.
Pro tip: Would cannot be used in the first conditional. The first conditional deals with realistic, likely future events—and would is reserved for the unlikely and the hypothetical. If you’re tempted to use would in a first conditional sentence, switch to will instead.
Quick Reference: Could, Should, and Would at a Glance
Here’s how the three modals map onto the four conditionals:
| Conditional | Situation | Could | Should | Would |
|---|---|---|---|---|
| Zero | Always-true cause and effect | No | No | No |
| First | Likely future prediction | Yes | Yes | No |
| Second | Unlikely / hypothetical | Yes | No | Yes |
| Third | Impossible (past situation) | Yes | Yes | Yes |
The key rule to remember: none of the three appear in the zero conditional, would never appears in the first conditional, and should doesn’t appear in the second conditional.
What’s Next
You now have a clear picture of how could, should, and would work in conditional sentences—and a table you can come back to whenever you need a refresher.
Grammar like this shows up throughout the TOEFL, especially in the Writing and Speaking sections, where you’ll need to discuss hypothetical scenarios and make recommendations. If you want to keep building your grammar and test skills, Magoosh TOEFL Prep offers practice questions and video lessons designed to get you ready for test day.
