I think I have just found the answer to, at least, one of the questions.
The sentences with the être verbs are both reflexive and, thus, do not require agreement. Is that correct?
Here is what you need to see:
1. Like adjectives, the past participle conjugated with être agrees in number (singular/plural) and gender (masculine/feminine) with the SUBJECT.
(This is the rule for NON-REFLEXIVE verbs.)
2. Past participles conjugated with "avoir" agree in number and gender with the PRECEDING DIRECT OBJECT (if there is one.) Elle nous a écrit. (nous is a preceding INDIRECT object = no agreement) BUT Elle nous a punis. (now nous is a direct object = agreement)
3. REFLEXIVE VERBS, always conjugated with "être" agree in number and gender with the PRECEDING DIRECT OBJECT (again, if there is one.) Ils se sont arrêtés. (se is the preceding direct object = agreement) BUT Elle s'est achêté des vêtements. (s' is the preceding indirect object = FOR herself. The direct object is actually "des vêtements." Ask yourself "What did she buy? and you get the direct object = des vêtements.)
Also, with the verb "parler" "Nous nous sommes parlé." = the 2nd "nous" is the indirect object = we spoke TO ourselves.
Now, let me check the examples you gave.
Nous nous sommes dit des histoires. = nous is TO ourselves/each other = indirect object. WHAT did we tell? "des histoires = following direct object. Get it so far?
Son idée a été rejetée = yes, it's with "avoir" but here the past participle functions as an ADJECTIVE = rejected idea. Past participles used as adjectives agree with the noun they modify = did you notice that the verb is still "être" although you see avoir because it is in the passé composé?
Où est la plante que tu as achêtée? = What did you buy? "la plante" which is the DIRECT OBJECT = preceding direct object or rule #2 above.
Indeed, this can be confusing. Condense it down:
1. passé composé with être = find SUBJECT
2. passé composé with avoir = find preceding DIRECT OBJECT
3. Reflexive Verb = find preceding DIRECT OBJECT
4. past participle used as adjective = agrees with NOUN(s) modified.
Do you feel less confused now? I wish you had asked BEFORE the quiz!
Sra (aka Mme)
Thank you so very much! I need to digest all this information now.
The reason I didn't ask before the quiz is that I thought I understood the passé composé. I found out the hard way that I didn't understand it completely.
Encore une fois, merci beaucoup de votre aide.