To know is to have an idea, understanding, or information about something. This is a very common verb in English.
These sentences and questions are in the present tense:
How many people do you know in your class?
Do you know what the capital of Belgium is?
Tony knows how to speak Spanish.
Vera doesn’t know the answer to the question.
The leaders of the city know what they want to do.
Not much is known about how the fire started. (This sentence is in the present tense, passive voice.)
These sentences and questions are in the past tense:
When Chen lived in Shanghai, he knew everyone on his street.
Did you know there was going to be a test today?
No one knew what to do for the man having the heart attack.
We didn’t know Shamso came from such a big family. (Now we know.)
I knew that was going to happen. I just knew it.
These sentences and questions are in perfect tenses:
Pedro and George have known each other since they were six years old. (present perfect tense)
How long have you known about their relationship? (present perfect tense)
Reena had never known who her birth mother was until just recently. (past perfect tense)
The word "know" is used with infinitives and gerunds:
We need to know more about what happened.
I want to know how to fix this.
Do you want to know the answer?
Knowing how to speak another language is a good skill.
Knowing that you have a problem is an important step towards dealing with it.
Knowing you are about to die can be a difficult experience for some people but a blessing for others.
The word "know" shows up in a few different expressions:
Well, what do you know? (The meaning depends on the situation. Sometimes it means I’m surprised! or it can mean I’m not surprised. You have to listen to the way a person’s voice sounds.)
She’s such a know-it-all. (She thinks she’s so smart and people dislike that.)
We were never in the know. (No one told us about this situation. It was secret.)
It’s been nice knowing you. (I’m never going to see you again. Good-bye.)
Note: The verb "know" is one of those verbs that is not ordinarily used in a continuous form. For example: "I am knowing many people," sounds very strange. It’s better to say, "I know many people." However, the word "knowing" is used as a gerund.