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

Would you confirm my understanding as to why there is no accent mark on the verbs of the 1st conjugation group in the 2nd person plural in the Pretérito imperfecto de indicativo?
The accent mark is used in the 1st person plural

hablaba hablábamos
hablabas hablabais
hablaba hablaban

I checked the pronunciation in Google translate and indeed "hablabais" carries the stress on the senond syllable.

The rule says that words ending in -s, n and a vowel must be stressed on the last but one syllable. And the accent mark is used in hablábamos to maintain compliance with the rule
Would you confirm my understanding that -ai- in the bais represents a falling diphtong thus it cannot be divided?

Yes, the 'ai' in the 2nd person plural is a falling diphthong in the case stated.

Should you encounter a verb like 'cambiar', which has a weak 'i' vowel before a strong 'a', you have a rising diphthong (the weak 'i' is also known as a glide). When this verb is conjugated in the 2nd person plural imperfect tense, the result is a triphthong, which must be accented on the strong 'a' vowel to retain the glide (ia): cambiáis.

(A Spanish triphthong always consists of a weak vowel (i,u), a strong vowel (a,e,o), and a weak vowel (i/y,u).)

Thank you. Your explanation of the triphthong structure is greatly appreciated.
So far I have encountered only four verbs that carry no accent mark in the 2nd person plural: ser, ir, dar, and ver. When I started perusing the chapter on the Pretérito imperfecto, the absence of the accent mark immediately attracted my attention.

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.
cambiais
https://i123.fastpic.org/big/2024/04...329f74e189.jpg
The resulting verb form has two syllables. In order to retain the stress on the penult, the accent is placed on "a" that results in "bias" being divided into two parts

cambiáis
https://i123.fastpic.org/big/2024/04...6f78b47a90.jpg
And, I think, this also holds true for verbs like "hablar", "ayudar" etc. We don't want a diphtong, we want two syllables to maintain the proper sillabication thus both as are accented.

habláis
aydáis
https://i123.fastpic.org/big/2024/04...fc5353c778.jpg
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.

As a follow-up to the preceding posts, I would request your opinion upon such verbs as "actuar", "continuar" etc.
The word "actuar" has a rising diphthong. It has two syllables and it is transcribed as
https://i123.fastpic.org/big/2024/04...c2c2fb2227.jpg The verb form in the 1st person singular is "actúo" with the accent on the penult
https://i123.fastpic.org/big/2024/04...2c89346ff1.jpg The conjugated forms are supposed to have their accents shifted one syllable forward (a.con.se.jar, a.con.se.jo) in the 1,2,3 p.sg and 3p.pl, which is not the case with "actuar" unless it is pronounced as "ac.tu.ar" in which case it violates the diphthong pronouncing rule: a catch-22 situation.
I would appreciate it if you could explain this point

Although the infinitive contains a rising diphthong, conjugated forms other than vosotros/vosotras and nosotros/nosotras get the stress (just like the corresponding forms in verbs that don't contain a rising diphthong). Only the plural first person and plural second person forms are stressed in the present tense.

Thank you. I can see the bias in my understanding now.