Hello. Don't worry, everyone struggles with the subjunctive, it is an area I have spent a lot of time worrying about myself! 

The subjunctive is merely a mood. It reflects how the speaker is feeling. It usually comes after phrases such as "Dudo que" "Es importante que" "Quiero que" - all of which express a feeling. 

For example: 

I want Will to go to Peru

Quiero que Will vaya a Peru

Whereas if I had just said - Will is going to Peru there would be no subjunctive as I am just stating a fact. 

I would then put this in a context through an article or video online to see whether the student can pick these up within a certain context. 

We would then speak for a bit trying to use as many subjunctives as possible.