Translation:I want to be certain that they have understood me.
Can anyone tell me why here we should use "capita" with the "a" at the end and not another form?
I assume you are asking why DL uses "capita" insted of "capito". That is because the person who speaks is of feminine gender and that is made clear by the use of "certa" instead "certo". If the person who speaks was
a man the sentence would be: "Voglio essere certo che loro mi abbiano capito"
If the subject were two or more men: "Vogliono essere certi che loro li abbiano capiti"
two or more women: "Vogliono essere certe che loro le abbiano capite".
When a compound tense is constructed with "avere", the past participle does not change according to gender or number
When the compound tense of a verb conjugated with "avere" is preceded by the third person direct object pronouns "lo", "la", "le", or "li", the past participle agrees with the preceding direct object pronoun in gender and number. The past participle may agree with the direct object pronouns mi, ti, ci, and vi when these precede the verb, but the agreement is not mandatory.
When "essere" is used, the past participle always agrees in gender and number with the subject of the verb.
So, are you saying that this sentence could have said "mi abbiano capito" instead and still have been correct? Just want to make sure that I understand. :)
In June of 2019 they now have a male voice saying this sentence which completely throws me off. If a male is saying it, it should end with "capito".
Very good points already made, but one thing was forgotten: It's about reflexive phrases. That is, e.g. MI abbiano capito/a. When reflexive, the phrase behaves like essere, changing the suffix.
Can anyone explain why this sentence requires the subjunctive? Is it because of the "I want"? Because without it, I'm pretty sure it would be "Sono certo che loro mi hanno capita."
The subjunctive is used because of “voglio essere certa,” but even after “sono certo” the subjunctive mood is more common.