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When I first started teaching, a student sent me an email saying “time flys so quickly in my class.” I realised that moment was the core of the problem: learners see the pattern “he/she/it” verbs adding “-s” and think “fly” + “s” = “flys”. But English doesn’t work that way. The spelling trap here is older than most learners realise, and once you understand the rule, you’ll never misspell it again.
The phrase “time flies” is one of those expressions that sounds so natural native speakers rarely think about it. But the correct spelling — “flies,” not “flys” — comes from a single, consistent rule about how English handles certain verb endings. I’ll show you exactly why the rule works this way, share the memory tricks my students use, and give you the confidence to write it correctly forever.
Key Takeaways
- Time flies, not flys — “flies” is the correct third-person singular form of the verb “to fly”.
- The Y rule — when a verb ends in consonant + Y, drop the Y and add “ies” (fly → flies, cry → cries, try → tries).
- Memory hook — think of other common verbs with the same pattern: “She cries,” “He tries,” “Time flies.”
- Common trap — learners mistake “fly” for a simple verb that just adds “s”, but English has this special rule for Y-ending verbs.
- “Time flies” idiom — the phrase means time passes quickly; it’s an idiomatic expression most native speakers use daily.
The Y-to-IES Rule: Why “Flies” Is Correct
English has a consistent rule for verbs that end in a consonant followed by the letter “y”. When you add a suffix like “-s” (for third-person singular) or “-ed” (for past tense), you must:
- Drop the “y”
- Add “ies” (or “ied” for past tense)
This rule applies to dozens of common verbs. Here are five side-by-side examples:
| Infinitive | Present tense (he/she/it) | Example sentence |
|---|---|---|
| fly | flies | Time flies when you’re having fun. |
| cry | cries | The baby cries every morning. |
| try | tries | She tries her best on every exam. |
| carry | carries | He carries his books in a backpack. |
| worry | worries | She worries about the presentation. |
Notice the pattern: the infinitive form ends in “consonant + y”. When you conjugate it for third-person singular (he, she, it), the “y” disappears and becomes “ies”. This is not random — it’s one of the most predictable rules in English verb conjugation.
Why “Flys” Is Incorrect
Many learners write “flys” because they see a simple rule at work: add “-s” to any verb. This works for most verbs:
- walk → walks (regular, just add “s”)
- play → plays (even though it ends in “y”, the “y” is preceded by a vowel, so just add “s”)
- jump → jumps (regular, just add “s”)
But “fly” breaks the pattern. The “y” is preceded by the consonant “l”, so the Y-to-IES rule applies. Writing “flys” ignores this rule entirely and is marked as incorrect in every major English dictionary and style guide.
The consonant-plus-Y rule: If a word ends in consonant + y, drop the y and add “ies”. If it ends in vowel + y (like “play”), just add “s”. This applies to all verbs with this pattern: fly, cry, try, dry, fry, pry, spy, apply, reply, supply, deny, defy, and many more.
The Difference Between “Fly” (Verb) and “Fly” (Noun)
English has a word “fly” that can be both a noun and a verb, which adds another layer of potential confusion:
Fly (noun)
Example 1: A fly landed on the table. (singular)
Example 2: There were dozens of flies buzzing around the kitchen. (plural — still follows the Y rule in noun form)
Fly (verb)
Example 1: Time flies. (present tense, he/she/it form)
Example 2: The eagle flies high above the mountains. (present tense, he/she/it form)
Example 3: Yesterday I flew to London. (past tense)
Example 4: She has flown to Paris three times. (past participle)
Whether you’re using “fly” as a noun (the insect) or as a verb (the action), the rule remains consistent. In third-person singular present tense, it becomes “flies”.
The “Time Flies” Idiom and Its Usage
The phrase “time flies” is idiomatic — it means time passes very quickly, usually when you’re enjoying yourself or busy. Native speakers use this phrase in everyday conversation, and it’s one of the most common time-related expressions in English.
Example 1 (casual): Can you believe it’s already December? Time flies!
Example 2 (reflective): I started this job five years ago — time really does fly when you’re doing something you love.
Example 3 (wistful): My daughter just graduated high school. Time flies; she was in kindergarten just yesterday.
Tip: The full idiom is often “Time flies like an arrow” (meaning time passes incredibly fast) or “Time flies when you’re having fun” (explaining that enjoyable moments feel shorter). Both use “flies,” not “flys”.
Common Spelling Mistakes and How to Fix Them
✗ Incorrect: Time flys so quickly that I forgot to call you.
✓ Correct: Time flies so quickly that I forgot to call you.
Why: “Fly” is a consonant-plus-Y verb, so it becomes “flies” in third-person singular present tense.
✗ Incorrect: The bird flys across the sky every morning.
✓ Correct: The bird flies across the sky every morning.
Why: Same rule. All verbs ending in consonant + Y follow this pattern.
✗ Incorrect: She cries easily, and the time flys when she’s upset.
✓ Correct: She cries easily, and the time flies when she’s upset.
Why: Both “cries” and “flies” come from the Y-to-IES rule. These verbs must be conjugated the same way.
Related Verbs That Follow the Same Rule
Once you master the Y-to-IES rule with “fly,” you can confidently conjugate dozens of other verbs. Here’s a short reference list:
- apply → applies
- carry → carries
- cry → cries
- deny → denies
- defy → defies
- dry → dries
- fry → fries
- hurry → hurries
- marry → marries
- pry → pries
- reply → replies
- spy → spies
- supply → supplies
- try → tries
- worry → worries
Sample Dialogue
Student: I keep writing “time flys” — I know it’s wrong but I can’t remember why.
Teacher: Think about the word “cry”. How do you spell the present tense “he/she cries”?
Student: Oh — c-r-i-e-s. Not “crys”.
Teacher: Exactly. “Fly” and “cry” both end in consonant plus Y, so they follow the same rule. Drop the Y, add “ies”. Flies, cries, tries — they all work the same way.
Student: That makes so much sense! I’ll remember that now.
Quick Quiz
Fill in the correct form of the verb in parentheses:
- Time _______ quickly when you’re learning something new. (fly)
- She _______ her hardest on every assignment. (try)
- The baby _______ when she’s hungry. (cry)
- He _______ every morning to the gym. (hurry)
- The bird _______ south for the winter. (fly)
Answers: 1. flies · 2. tries · 3. cries · 4. hurries · 5. flies
Related Articles
- ↑ Master Pillar: English Grammar
- Anytime vs. Any Time — when to use one word or two
- ↑ Back to pillar: English Confused Words (Pillar)
Frequently Asked Questions
Why is it “time flies” and not “time flys”?
English has a rule called the Y-to-IES rule: when a verb ends in consonant + Y, you drop the Y and add “ies” in the present tense (he/she/it form). “Fly” ends in consonant + Y (the L before the Y), so it becomes “flies,” not “flys.” The same rule applies to verbs like “cry,” “try,” and “carry.”
Is “flys” ever correct in English?
No. “Flys” is never a correct spelling in standard English, whether as a verb or a noun. The only correct form is “flies,” which works as both the plural noun (multiple insects) and the third-person singular present tense verb.
What other verbs follow the same rule as “fly”?
Any verb that ends in consonant + Y follows the same rule. Common examples include: cry → cries, try → tries, worry → worries, carry → carries, marry → marries, apply → applies, reply → replies, and deny → denies.
Does the Y rule apply to the past tense of “fly”?
The past tense of “fly” is “flew” (irregular), not “flied.” However, in baseball, “flied” is sometimes used (e.g., “He flied out to left field”), though “flew” is still acceptable. For standard English, “flew” is the past tense and “flown” is the past participle.
What does the phrase “time flies” mean?
The phrase “time flies” (or the longer version “time flies like an arrow”) is an idiom meaning that time passes very quickly. It’s often used to reflect on how fast days, months, or years go by, especially when you’re busy or enjoying yourself.
How do I remember the Y-to-IES rule?
Think of a pattern word like “cry” → “cries” or “try” → “tries.” Once you can spell those correctly, you know that “fly” follows the same rule: “flies.” Grouping verbs by their pattern helps you remember the rule instead of memorizing each one individually.
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