Well, SHOULD we, or SHOULD we NOT?

I'm sure you're familiar with the phrase 'tiene que', meaning 'you have to'. 
Another common way of saying 'you should' or 'you must' is to use the verb deber. 
So, consider the following sentences:

'Tienes que manejar, no tienes que viajar por avión' 
'Debes manejar, no debes viajar por avión' 

Apart from the subtle difference between 'you have to' and 'you should', normally implied by the choice of verb, can you see any difference? 

You may have spotted it immediately, but it's worth pointing it out, just so that you're aware of the structural difference between the two forms. 

The first says
'You have to drive, you DON'T HAVE TO fly' 
While the second says
'You must drive, you MUST NOT fly' 

The point here is that 'tener que' and 'deber' are very similar in a positive context, but quite different in the negative. 

The difference extends across all tenses, so you could say
'No habría tenido que pagar'-' you should not have had to pay'
'No habría debido pagar' - 'you should not have paid' 

I hope that's solved another problem you didn't even know you had. 

¡Feliz Navidad!