Understanding when to use 'tu sois' (être imperative)

Question

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.

Answer 1

Tu sois is not an imperative form, but a subjunctive form. The subjunctive mood is very frequent in French for verbs in subordinate clauses that express feelings such as a doubt, a wish, etc.

See here.

Answer 2

The subjunctive must be used after verbal phrases expressing feelings (subjunctive).

• être: content/ heureux/ désolé/ triste/ étonné/ surpris /avoir peur/ avoir honte/ craindre/ regretter/ se réjouir

Il est dommage / Il est honteux / Il est regrettable / Il est surprenant/ Il est triste

• Il est dommage que tu ne puisses pas comprendre, mais l'année prochaine ça ira mieux, tu comprendras déjà beaucoup.

If the use of the subjunctive appears irrational, do not hesitate and use another form, as for instance this one.

• Elle est triste de te savoir malade.