To "know" is to have a definite (without a doubt) knowledge of something. To "think" (as in the same context as "know") is more of an indefinite answer; another meaning of "think" is a thought process "im THINKING". To "believe" can be used like in the same way as "think" (an indefinite answer). Another meaning of "believe" is to have faith without seeing, like a faith in religion. "consider" in the context of "I'm considering these apples" means to take regard (like a top choice about something). hope this helps. sorry if it didn't