"I have received a pair of them."
Translation:Ne ho ricevute un paio.
I guess it's because "ne" replaces something plural (un paio [di something plural]), so the participle has to agree with whatever its replacing. But like you say, there's no way of telling whether it's masculine or feminine.
Persona 1: Hai ricevuto un paio di libri?
Persona 2: Sì, ne ho ricevuti un paio.
1: Anche hai ricevuto un paio di maglie?
2: Sì, pure ne ho ricevute un paio.
There is no way for us to tell what has been received but, - when 'ne' (of them) is used in 'passato prossimo' the ending of the past particle 'ricevuto' (received) should reflect the number and gender of the object.
Possible object --> Translation
'i libri' p.m. --> Ne ho ricevuti . .
'le chiave' p.f. --> Ne ho ricevute . .
'i pantaloni' s.m. --> Ne ho ricevuto . .
As Emile mentioned correctly, i pantaloni is NOT singular, as you can see from the use of the masculine plural article 'i' instead of singular 'il'. I pantaloni is one of these words that take the plural even when talking about just one pair. (presumably because one pair has two legs ;) )
The word pantaloni is indeed masculine plural, however un paio di pantaloni (a pair of trousers) is singular (one group of two things), so
"ho ricevuto un paio di pantaloni" would be correct, if a pair of trousers was the object of the sentence.
However, when you substitute the things with "ne" to mean "of them", you're no longer referring to the pair as the object, but the individual things themselves (the trousers, plural), so
"ne ho ricevuti un paio" would be the correct construction, when the 'them' referred to by ne were trousers.
We know that ne means of them in this case, because it's the only thing that makes sense with "I have received a pair". You wouldn't say a pair of it, a pair of some, a pair of any, or a pair of none.
This exercise complicates things because it is a sentence fragment without specifying what the 'them' are, so all you see is "un paio", with no obvious connection as to why the plural form of ricevuto is expected.
The forum discussion on the companion exercise to this one (Italian to English) has a more detailed explanation by Signor Formica.
Some assistance with grammar, per favore. :-) My understanding is that when the 'helper' verb is avere, the participle does not agree with the subject or number (whereas if the helper verb is essere, the participle does agree with the gender and number. So: ho ricevuto, hai ricevuto, ha ricevuto, abbiamo ricevuto, avete ricevuto, hanno ricevuto. So why is ricevute (presumably the feminine plural) accepted when the helper verb is based on avere? Thanks!
Since others may care, I'll answer even though this was asked a year ago.
When the helper verb is essere then the participle must have agreement (in gender and number) with the subject.
When the helper verb is avere then the participle must be masculine singular unless there is a direct object before the verb (ne in this case). If a direct object precedes the verb, then the participle must have agreement with the direct object.
Both ricevuto (which is what I put) and ricevute is accepted. I am not sure why either is considered correct...it should be one or the other but I put ricevuto as I think of "pair" as singular which is how it is considered in English. I assume ricevuti would be correct also if the pair of whatever is being spoken about is masculine.
The noun: un paio, of them, for them, of pants etc. are descriptive of the object of the sentence; "un paio". But according to what i read here there seems to be some ambiguity on which form of the p.p. to use. Duo, and perhaps the language is permissive on this point?
While "a pair" in English may be considered singular, it describes a quantity of two, which is plural.
"When ne is used in the past to replace a noun used with an expression of quantity, the past participle agrees in gender and number with the noun ne is replacing."