To know something means to have information, understanding, or awareness about it in your mind. When you know your best friend's birthday, the capital of France, or how to ride a bike, that knowledge lives inside you, ready to use.

Knew is the past tense of know. It means you had that knowledge or awareness at an earlier time. If you knew the answer to a question yesterday, you had it in your mind then. When someone says “I knew it!” after something happens, they're saying they had figured it out or suspected it beforehand.

Knowledge comes in different forms. You might know facts like multiplication tables or state capitals. You might know people, meaning you're familiar with them and understand their personalities. You might know how to do something, like swim or play chess. Each type represents information or understanding you've gained through learning, experience, or acquaintance.

When you say “I knew that already,” you're indicating the information wasn't new to you. When you knew better than to touch a hot stove, you had the wisdom to avoid it.