Dear Maryam, I think you wanna ask about the verb fill . 1- If you go somewhere or want to apply for a job and they want you to supply them with some information, you need to fill out a form or you need to fill it out. 2- but when you are run out of gas, you will go to a gas station and ask to fill up your car. You may say: Fill it up with regular. 3- sometimes you need to fill in some blanks in an exam then you will be told to fill in the blanks with proper verbs/ adj/nouns..... I hope this will help you. :)