Learn Spanish (Latin America): Lo triste es la impotencia cuando uno no tiene con qué comunicarse. - The sad thing is the helplessness when one has no means to communicate.

This sentence uses the structure no tener + preposition + qué + infinitive. The preposition con is required because you communicate with a device or means (comunicarse con algo). So, no tiene con qué comunicarse literally means 'one doesn't have anything with which to communicate.'

Why does qué have an accent mark in con qué?

In these negative structures (no tener or no haber + preposition + interrogative word), the question word acts like an embedded indirect question. It translates roughly to 'doesn't have what to communicate with,' so qué keeps its interrogative accent.

Why is comunicarse in the infinitive instead of the subjunctive?

You use the infinitive (comunicarse) when the person who 'doesn't have' is the same person doing the action. Here, uno is both the person who lacks the means and the person trying to communicate. If the subjects were different, you would use the subjunctive.

How does lo triste work? Why not use el triste?