Should and Must

I am not sure if this is the right place to open this topic but I have a question about using of should and must.

I tend to use have to and should but in some cases the native speakers use “must”. I know if I google it I can find a lot of exercises and sentences about it. However I want to learn here. I know that many of us think that forums are very active and this is not a good signal for the website.

I found a map for blind people. It “must” be developed before the World War. - Is this sentece correct or can I say “It should be developed before the WW1.”

I googled but could not find such a sentence like;

It should have been developed - does it sound too freaky?

The difference between MUST and SHOULD.

MUST is used when expressing obligation or an unavoidable requirement, whereas SHOULD is more of a recommendation, or simply a desirable goal.

Similar to ‘have to’ – strong obligation.

  1. You must abide by the law.
  2. You mustn’t smoke indoors, it is illegal.
  3. You must arrive on time; otherwise, we will leave without you.

Similar to ‘ought to’ – a suggestion.

  1. You should try to get some sleep; it is very late, and you have a lot of work to do tomorrow.
  2. You shouldn’t be so mean to your brother, he’s just trying to help you.
  3. You should visit us more often, we hardly ever see you nowadays.

Georgia’s explanation is a good starting point in general for ‘must’ and ‘should’. However, based on your example sentence, your question is really about the past modals ‘must have’ and ‘should have’. If I understood you correctly, the correct form of your example sentence is this:

“I found a map for blind people. It must have been developed before WW1.”

It uses ‘must have’ as a modal of deduction. It means that the person isn’t 100% certain that the map was developed before WW1 (they have no proof), but it seems to be the only logical possibility.

On the other hand, the past modal ‘should have’ is for regret, e.g. “I should have studied more last week” (which means, “I wish I studied more than I did,” or “It would have been better if I studied more than I did”). So you can’t use ‘should have’ in your example, unless you completely change the meaning of the sentence, to say something like, “They should have developed more maps for blind people before WW1.”

I hope this helps!

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You are right.
But the sentence ‘It must have been developed before WW1’ denotes that you are quite sure that it was so in fact.
If you doubt you’d rather say: ‘It may have been developed before WW1’

You are indeed quite sure but it’s still by deduction. If you know 100%, you can just say “It was developed before WWI.” The use of “must have” means that you have eliminated any other possibility, leaving this one as the last remaining one, so it “must” be true.

Let me change examples to remove any doubt. If my wife tells me she is going upstairs, and then our house guest asks me where she went, I would say, “She went upstairs.” If she never told me where she was going, and the guest asked me where she went, and I checked all the downstairs rooms and couldn’t find her, and didn’t hear her leave the house, I would say, “She must have gone upstairs,” because there doesn’t seem to be any other possibility. I don’t know for a fact that she went upstairs but I can’t think of any other place where she could be.

You understood correctly. All I wanted to learn was that. “It must have been developed before WW1.”