Auxiliary verbs fall into several classes. The first, and most important, in a pattern that English shares with most European languages, are 'to be' and 'to have'. These are the central auxiliary verbs. Along with 'to do', these three are known as the primary verbs, so important are they.
For example, "She was ill the other day" is the simple past of the (semantic) verb 'to be'. In "she was singing", on the other hand, was is an auxiliary which is used to form the continuous past active of the verb 'to sing'. This is 'to be' being used as an auxiliary to form the continuous aspect. In "the referee was kicked by a player", the 'was' shows us a passive 'to be' used to make a past passive tense. These examples show us again that in the study of grammar there are no simple answers that can be learnt by heart. Students have to use their intelligence, judgement and experience to analyse how the words are being used.