---Hi. Lucy! What’s wrong with you? You look so worried.---I have many problems to solve, but I don’t know ___________ first.
It's very dangerous __________ football in the street.
-- ________ you tell me how to write it in English?-- I'm sorry, I can't.
Look! They ________ in the hallway.
Can you help me ________ my homework?
The teachers ________ on the blackboard with chalk in the future.