The affirmative past tense in Japanese is formed by adding ました to the root stem of the verb. The best way to form the past tense is by remembering the formation of the present tense of the verb, leave off the ending, and add ました. For example, the present tense affirmative of かえる (to return) is かえります. Take off the ます and add ました to the stem. You get かえりました as the final product. Here are some more examples:

Infinitive: (はいる)(入る) to enter

Present, polite, affirmative: (はいります)

Past, polite, affirmative: (はいりました)

Infinitive: (する) to do

Present, polite, affirmative: (します)

Past, polite, affirmative: (しました)

Infinitive: (つくる)(作る) to make, build, create

Present, polite, affirmative: (つくります)

Past, polite, affirmative: (つくりました)

Infinitive: (いそぐ) (急ぐ) to hurry

Present, polite, affirmative: (いそぎます)

Past, polite, affirmative: (いそぎました)