Beginner here! I am trying to understand why we use 'tu sois' (imperative form of être) rather than just 'tu es' in some contexts.
For instance, I have come across the sentence:
"Elle est triste que tu sois malade." (She is sad that you should be ill)
But in English, it would be more natural to just say "She is sad that you are ill". And I cannot see why this should be an imperative mood. The person is already objectively ill and your not asking them to be otherwise, so it seems very different to, for instance, "Ne sois pas idiot", "Sois ma valentine" or "sois reasonable" where you are asking something of the other person and also giving a subjective interpretation of their behaviour in two of these.