According to my teacher all of the following sentences are wrong. The test was given regarding modals and prepositions.Look: 1:I walked on the road. (She says across is correct instead of "on") 2:The ball rolled on the floor. (She says off is correct instead of "on") 3:You must submit your project tommorow.(She says shall is correct instead of "must") 4:You should obey your elders. (She says must is correct instead of "should") NOTE:I used "should" sentence 4 caz I thought we cannot compelling anyone to obey elders. Plz tell me what's right.

closed as off-topic by Dan Bron, Mick, Nigel J, tchrist Nov 24 '17 at 13:26

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All your sentences are grammatically correct, though they differ in meaning from your teacher's versions. eg: "Did you walk on the road, or on the pavement?"

"I walked on the road."

"How did you get to the café?"

"I walked across the road."

"The ball rolled on/off the floor."

'Off' implies it went from the floor to some other surface; both are grammatically correct.

"You shall/must submit your project tomorrow."

Both mean an imperative; 'must' is more idiomatic in contemporary usage - 'shall' sounds a little archaic. [Incidentally, it still has a specific use in formal specifications etc, where 'shall' indicates that this is a mandatory requirement of the spec.]

"You should/must obey your elders."

You are correct that 'must' implies a stronger obligation than 'should'; both are grammatically correct.

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