The verbable is not a verb but an adjective.
Thus, the verb preceding the verbable is not an auxiliary verb! It is, in fact, a link verb.
The verbable is usually preceded by the link verbs blijven, vallen or zijn (these are all irregular verbs).
| blijven
| to stay
| het blijft te proberen
| it remains possible to try
|
| vallen
| to fall
| het valt te proberen
| it is possible to try
|
| zijn
| to be
| het was te proberen geweest
| it had been possible to try
|
The statement that we are not dealing with an auxiliary verb here must be rather confusing as the construction clearly consists of 'te' and a verb. The last example shows us that zijn (in the pluperfect: was geweest) is not an auxiliary verb. Had it been an auxiliary verb for proberen, the past participle would have turned into an infinitive.
Questions? 
