What is the subjunctive mood?

The subjunctive mood is used to indicate uncertainty, or possibility, in french. For example you would use the subjunctive mood when talking about your feelings, judgments, doubts and opinions, and when ordering someone to do something. Certain expressions followed by que or qui require the subjunctive, for example 'il faut que tu fasses tes devoirs,' or 'il est important que tu fasses tes devoirs'. In the first situation, you are ordering someone to do something: 'you must do your homework' and in the second situation you are expressing your opinion: 'it's important that you do your homework'. Because of this, both sentences require the subjunctive. Note that 'Il est important de faire tes devoirs' does not require the subjunctive as it uses de instead of que. Some exceptions such as 'de maniere que' require the subjunctive too.