Select all the rules that apply to root changes in the past tense. A) Stems of infinitives -er and -ar do NOT change.

B) Stems of infinitives -ir change in the third person, singular and plural.

C) If there is only one vowel in the stem change in the present, the same change is preserved in the past tense.

D) If there are two vowels in the stem change in the present, the change in the past consists of the first letter of the change.