When forming the past negative polite tense, add the ending ませんでした to the conjugated stem of the verb. It’s helpful to know the present polite negative, because knowing how the present polite negative endings can help form the past polite negative. For example, the verb しゃべる (to chatter) in the present polite negative is しゃべりません. To form the past polite negative of しゃべりません, just add でした. Now if you can’t remember how to form the present polite negative, you can also use the present polite affirmative and add ませんでした to しゃべり and drop ます. Though, I do think the present negative is easier.

Here are some examples:

Infinitive: iru (いる) to need

Present Polite Negative: いりません

Past Polite Negative: いりませんでした

Infinitive: asobu (あそぶ) to play

Present Polite Negative: あそびません

Past Polite Negative: あそびませんでした

Infinitive: shinu (しぬ) to die

Present Polite Negative: しにません

Past Polite Negative: しにませんでした