“To know” means to have knowledge or be familiar with a person, fact, or skill; the sentence shows which sense you mean.
If “to know” feels simple, you’re not alone. Then you hit a sentence like “I know him” vs “I know about him” and the meaning shifts. This page pins down the definition, shows the main grammar patterns, and gives you quick tests you can use while writing or speaking.
Definition Of To Know In Plain English
“To know” has two core ideas. One is having a fact in your head. The other is being familiar with a person, place, or thing through experience. English uses the same verb for both, so the words around it carry the load.
When you want a clean definition, try this: to know is to have knowledge, familiarity, or skill that you can recall and use. The next step is picking the right pattern for the sense you want.
| Sense Of “Know” | Common Pattern | Sample Sentence |
|---|---|---|
| Facts and information | know + that / know + wh-word | I know that the class starts at 9. |
| Familiarity with a person | know + person | I know Dr. Kaya from my old school. |
| Familiarity with a place or thing | know + place/thing | She knows this campus well. |
| Skill (“know how”) | know how to + verb | They know how to format a report. |
| Awareness (“know about”) | know about + topic | I know about the deadline change. |
| Recognition | know + by / know + from | I know her by her voice. |
| Certainty in a moment | know + (object) + to be | He knew the answer to be wrong. |
| Social connection | know + each other | We’ve known each other since 2018. |
Quick way to pick the right sense
Ask yourself a blunt question: is this about facts or familiarity? If you can swap in “be aware of,” you’re near the facts side. If you can swap in “be acquainted with,” you’re near the familiarity side. If you can swap in “be able to,” you’re in “know how.”
How “To Know” Works in real sentences
English learners often learn “know” early, yet the tricky bits live in the add-ons: that-clauses, wh-clauses, prepositions, and time words. Once you see the patterns, you can build cleaner sentences fast. In many drafts, the definition of to know is fine, and the pattern is what needs a fix.
Knowing a fact: “know that” and “know wh-”
Use know that when you state the information directly. Use know plus a wh-word when the information is framed as a question inside the sentence: who, what, where, when, why, which, or how.
- I know that this chapter is on verbs.
- Do you know where the library is?
- She knows why the score changed.
In speech, “that” often drops: “I know you’re busy.” In formal writing, keeping “that” can reduce confusion when the sentence is long.
Knowing a person: “know someone” vs “know of someone”
“I know someone” means you’ve met them or have a real relationship. “I know of someone” means you’ve heard the name, yet you don’t have a direct connection. “I know about someone” usually points to information about them, not a relationship.
- I know Lina. (You’ve met.)
- I know of Lina. (You’ve heard the name.)
- I know about Lina’s research. (You know details.)
Knowing a place or topic: “know” vs “know about”
“I know Paris” suggests experience: you’ve been there, you can get around, you’ve got memories tied to it. “I know about Paris” means you have information, maybe from reading or class. Both can be true, yet they’re not the same claim.
The same split shows up with topics:
- She knows statistics. (She can work with it.)
- She knows about statistics. (She has some information.)
Know how: skill, not information
“Know how” points to ability. It often shows up with a verb: know how to write, know how to fix, know how to log in. A quick check is whether you can replace it with “can” without changing the meaning too much.
- He knows how to cite sources.
- They don’t know how to merge the files yet.
Common word partners and prepositions
Prepositions after “know” feel small, yet they steer meaning. Learning them as mini-chunks helps you avoid awkward phrasing.
Know about
Know about is awareness of a topic, event, or detail. It can be deep or shallow; the phrase alone doesn’t promise depth.
- I know about the policy change.
- She knows about that author’s work.
Know of
Know of is lighter than know about. It often means you recognize a name or existence, not the details.
- I know of that school, yet I’ve never visited.
Know as
Know as gives a label: a nickname, role, or identity. It’s common in biographies and academic writing.
- The city is known as a port.
- She’s known as “Mina” at work.
Know by and know from
Know by and know from express recognition: you identify something using a clue.
- I know that song by the intro.
- I know her from class.
Definition Of To Know For Writing And Speaking
When you write, “to know” can sound too strong or too vague, depending on the claim. Tightening the sentence keeps your tone honest and your meaning clear.
Pick a verb that matches your evidence
If you have direct proof, “I know” can fit. If you only have a guess or a hunch, pick a softer verb.
- Stronger: I know the data set is incomplete.
- Softer: I think the data set is incomplete.
- Neutral: It seems the data set is incomplete.
Keep “I know” out of formal statements when it adds nothing
Many essays get cleaner when you remove “I know” and state the point directly. Compare these pairs:
- Wordy: I know that photos help memory. Cleaner: Photos can help memory.
- Wordy: We know that the test was hard. Cleaner: The test was hard.
In research writing, “I know” can sound like you’re asking the reader to trust you without evidence.
Use “known” in passive voice with care
“It is known that…” can sound stiff. If you use it, pair it with a clear source. The Cambridge Dictionary entry for “know” lists the main senses and common patterns.
Time forms that change meaning
“Know” behaves like other stative verbs most of the time. That affects tense choice, especially the continuous form.
Present simple for stable knowledge
Use the present simple for facts you hold now: “I know the answer.” “She knows French.”
Past simple for when the knowledge was in the past
Use past simple when the knowledge or familiarity existed at a past time: “I knew him in college.” “They knew the rules last year.”
Present perfect for a connection that started in the past
Use present perfect with since or for to show duration: “I’ve known her since 2020.” This is a common spot for mistakes because learners want a continuous tense.
If you write “I know her since 2020,” readers will hear a grammar bump. Switch to “I’ve known her since 2020.” The verb “know” marks a state, so the perfect tense carries the time span cleanly. In speech, you’ll hear it often in class too.
Why “I’m knowing” usually sounds off
“Know” usually doesn’t take the -ing form in standard English. “I’m knowing the answer” feels wrong to most readers. Use “I know the answer.”
Common mixups and quick fixes
This is where most writing slips happen: mixing up “know” with “learn,” using the wrong preposition, or choosing the wrong verb for the kind of knowledge you mean.
| Mixup | Why It Sounds Off | Cleaner Option |
|---|---|---|
| I know to swim. | English uses “know how to” for skills. | I know how to swim. |
| I’m knowing him. | “Know” is usually not continuous. | I know him. |
| I know him since 2020. | Needs present perfect for “since.” | I’ve known him since 2020. |
| I know about her. (meaning you met) | “Know about” is info, not acquaintance. | I know her. |
| I know her. (meaning you heard the name) | Sounds like a relationship. | I know of her. |
| I know the book is. (broken clause) | Clause needs a full statement. | I know the book is on the desk. |
| I know what is he doing. | Indirect questions keep statement order. | I know what he’s doing. |
| I know to the answer. | No preposition needed. | I know the answer. |
Know vs learn
Use learn for the moment you get new knowledge. Use know for the state after you’ve learned it.
- I learned the rule yesterday. Now I know it.
Know vs understand
You can know a fact without understanding it. “Understand” points to grasping the logic or meaning. In essays, “I understand” can fit when you explain a concept in your own words.
Mini checks you can run while editing
When you revise a draft, use these fast checks to make sure “know” is doing the job you want.
- Relationship test: If the object is a person, ask “Have I met them?” If not, try “know of” or “know about.”
- Skill test: If the object is an action, switch to “know how to.”
- Evidence test: If you can’t point to a source or experience, swap “I know” for “I think” or rewrite the sentence as a neutral statement.
- Tense test: With “since” or “for,” use “have known.”
- Clause test: After “know,” check that the clause is complete: subject + verb.
Where “to know” shows up in school tasks
In learning settings, “know” often appears in rubrics and learning targets: “Students know the steps,” “Students know the terms.” If you’re writing your own study notes, turn “know” statements into actions you can check.
- Instead of “Know the formula,” write “Solve five problems using the formula.”
- Instead of “Know the terms,” write “Define each term in one sentence.”
- Instead of “Know the dates,” write “Place the dates on a timeline.”
If you want a second reference for meaning and usage, the Merriam-Webster definition of “know” breaks the verb into senses and shows common constructions.
Practice prompts for cleaner sentences
Do this in ten minutes. Write six lines: three with “I know that…”, three with “Do you know where/when/why…”. Then write three lines with “I know how to…”.
Last pass checklist before you submit
Use this as a final sweep when “know” appears in an essay, email, or homework response:
- Each “know” statement matches what you can prove, show, or explain.
- People: “know” means you’ve met; “know of/about” means you’ve heard or read.
- Skills: “know how to” is in place.
- Indirect questions keep statement order (“I know what he did”).
- With “since/for,” you used “have known.”
If you remember just one line, make it this: the definition of to know stays steady, yet the pattern you choose tells the reader what kind of knowledge you mean.