Hola Whatit

In our lesson there is no section called "variable subjunctive" nor a sentence like the one you mentioned. Can you be more specific about where you saw that sentence in our site please.

Gracias y saludos

Hola Whatsit and Alan

I didn't realise this came from Lawless, sorry. 

You can use both the indicative and the subjunctive in this case, depending on what you want to convey.

With the indicative:

No tengo un profesor que vive en Madrid.

Here, using the indicative, indicates that you are "declaring" something, probably something that has been mentioned before and you are simply "declaring/stating" that fact. This would have been a sentence used for example if someone had previously said to you "Hey, you have a teacher that lives in Madrid, right? - then you answer: No, yo no tengo un profesor que vive en Madrid. The indicative is justified here too because the reference, in this case "el profesor" is seen as someone specific in the mind of the speakers - identified.

With the subjunctive, (and this is more common to hear):

"No tengo un profesor que viva en Madrid." 

Here, the person, more than declaring a fact, or referring to a "specific person", is saying that he doesn't have any teacher who may be living in Madrid, the teacher (whoever that may be) is seen as someone unknown, unidetified. 

So, your assumption is right, there is no teacher he knows of who lives in Madrid. 

Saludos cordiales

Inma