Should & Must in Spanish

Should & Must in Spanish
 

A native Spanish speaker said he would use the following translations

"Debo haber dejado el libro..." I must have left the book...
"Debí haber dejado el libro..." I should have left the book...

That is not immediately obvious to me that the different tenses would be translated thus in English.

The present tense 'debo haber dejado' means 'I must have left'. Here, it conveys supposition instead of obligation.

Many people prefer 'debería haber dejado', which means 'I should have left', but it is also OK to use the preterite and/or imperfect to get the same meaning. These forms convey obligation - something you ought to have done.

Does that help?

Must and should and tenses
 

The word "should" is from an old Viking word that conveys both obligation, and comes from the goddess of the future. So it is logical that the future tense would be used in a romance language.

I am just not clear why the past tense would convey the same thing. It is also not obvious why the present tense would mean supposition.

I recognize the general problem of conveying the meaning of English modal verbs in a language which expresses mood via conjugations.

In 501 Spanish verbs, in their introduction to future tense uses this example:
Maria estará enferma
Mary must be sick

which confuses me even more.

There are a couple of web sites I found that may clear up your questions. Google 'deber ought should must' to find them.