The principal part which never uses an auxiliary to form a tense is the(past, past participle, present) part.

The answer to this question is : The principal part which never uses an auxiliary to form a tense is the PAST part

PAST.

Explanation:

A verb has four principal parts or basic forms that are used to form the different verbal tenses, here is a brief explanation of these parts:

The present (or infinitive) part is used to express the present and the future time. I walk and I will walk. As you can see, in the future tense, it does use an auxiliary: will.The present participle forms the progressive forms of the present, the past and the future: I am talking, I was talking, I will be talking, etc. In all forms, this part uses the auxiliary verb be or will.The past participle forms the perfect tenses: the present perfect, the past perfect, and the future perfect (I have talked, I had talked, I will have talked). Thus, this part uses auxiliaries: have and will.The past part is used to express the past tense only, and it is the only part that does not use auxiliary. Here are some examples: I talked, I smiled, I bought, I thought.