You seem to be confused about terminology: a MAIN verb is a finite verb, either the only or the first element in the verb phrase of a sentence, illustrated respectively by 'is' in
 He IS a doctor.
 The house IS cleaned every day .
Thus a main verb contrasts with a NONFINITE verb, such as 'being' in
 The food is BEING cooked.
The distinction to which you seem to be referring, however, is that between FULL verb and AUXILIARY verb, a quite different matter. When 'be' (or any form of it, finite or nonfinite) is complemented by a participle, it is termed an auxiliary verb, as in  and  above. Otherwise it is a full verb, as in .
Thus, as a main (or 'finite') verb it may be either full or auxiliary, and as a nonfinite verb it may equally be either full or auxiliary.