To assume is a regular verb that means to suppose something to be the true, but without proof.

For example: I assume you’re here to learn English.

To know is an irregular verb that means to be absolutely certain or sure about something, usually through observation, inquiry, or information.

For example: I know I should practise English every day, but I never seem to have the time.

To think is an irregular verb that means to have a particular opinion, belief, or idea about someone or something.

For example: I think English is a global language.

!Note – If you need to remember which one to use, memorise this sentence:-

Why do some people assume they know what other people think about something?