Is were an auxiliary verb or a main verb?

WERE can be both an auxiliary verb and a main verb.

WERE is the past simple form of the verb BE.

Here are examples of the verb BE used as a main verb.

e.g., Usually, they are happy.

Yesterday, they were happy.

An auxiliary verb is a helper; it helps the main verb in the sentence.

Here are examples of the verb BE used as an auxiliary verb. The main verb is WORK.

e.g., Usually, they are working when I phone.

Yesterday, they were working when I phoned.

I hope that helps.