Questions about example sentences with, and the definition and usage of "Umm"

"umm" can be used mostly when referring to confusion. For example: "umm... excuse me, can you tell me where the station is please?" "hmm" can be used mostly when referring to a decision. For example: "hmm.. I don't think that is the right answer"

"He must have looked the wrong way because a car was coming when he stepped onto the road" "I shouted but the boy couldn't hear me-he didn't stop. I waved at the car but the driver didn't notice me either" "At the last moment, the boy must have seen the car because he suddenly jumped out of the way" "The car could have killed the boy!" "I saw him fall over, so it must be (must have been) his arm" "Thank you, you must have been in shock to see this happen"