The words now and know are not interchangeable. Now represents a section of time which is in the present, while know implies a sense of understanding about something—having known or knowing means that the information is clear in your mind. Now can serve a plethora of roles as an adverb, conjunction, adjective, and noun. Now excludes anything that happens in the future.
Now can also mean there is a sense of urgency, and something needs to happen at once. Know could also suggest something that may have taken place in the present and can be accusatory, as in “I know what you did.” Know is a verb, while now it is an adverb. Know is also to be friends with somebody and understand them. Know and now are both two of the most popular words used in the English language.