Both are modal auxiliaries and should is not just the past of shall.
Here is a good explanation: https://learningenglish.voanews.com/a/everyday-grammar-should-vs-shall/3107315.html
Wegen viel Arbeit oder wegen vieler Arbeit?
English speaking practice
The difference between "has + been + p.p" and "had + been + p.p"
Is anyone here to practice English with me?
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