No accent mark for the 1st conj verbs in the 2nd pers pl. in the Pretérito imperfecto

[Rusty]

Originally Posted by Nfqufktc

Speaking of “cambiáis”, allow me to check my understanding again:
Written as “cambiais” (without the accent mark), “iai” is a triphthong which is a single syllable.
An accent mark MUST APPEAR on the strong vowel in a triphthong.

And, I think, this also holds true for verbs like "hablar", "ayudar" etc. We don't want a diphthong, we want two syllables to maintain the proper syllabification, thus both a's are accented.
Is this reasoning valid?

Simply put, no.
These last verbs, in the present tense second person forms, all contain an accent mark because the last syllable is stressed. It has nothing to do with whether a diphthong appears in the last syllable or not. This is true for all present tense second person forms (-AR, -ER, and -IR) for vosotros/vosotras.
Especially have a look at the -IR form, whose conjugated second person plural form in the present tense is 'ís': vivís. There is no diphthong there, but it goes to show that the last syllable is stressed in the second person plural present tense conjugations.
Again, when using vosotros/vosotras, the present tense verb ending is the stressed syllable. You can look at nosotros/nosotras endings, as well. The stressed syllable there is the 'a', 'e', or 'i' of the ending ('amos', 'emos', 'imos'). All of the other persons do NOT take the stress in the present tense.