The subjunctive is a different kind of tense because it doesn’t exactly exist in English, so- soyez courageux (be brave!)!
It is used in certain situations with specific preceding verb phrases. It shows obligation or necessity, but it can also be used with expressing doubt, and other things like emotions.
“Il faut que” is always followed by the subjunctive, for example.
“Il faut que tu fasses les devoirs” = You must do your homework! (or “It is necessary that you do your homework”)
Skim through Lesson 35, specifically pp. 508-509, to get an idea of the tense.
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