To complement other answers given, please note that "y" is a neutral pronoun that replaces a noun or the name of a place previously mentioned.
Therefore, "elle y est allée hier" means "elle est allée hier (à l'école, à Paris, etc)".
Gramatically, "y" serves as the complement of the sentence while "elle" is the subject. Être verbs like "aller" only agree with the subject and not the complement or object, so "aller" has to be conjugated according to "elle" whether or not the "y" is there.
And like Sitesurf said, "y" is considered neutral even if it replaces a complement that contains a feminine noun like "à la bibliothèque".
Couldn't "y" be male or female? Or is allée reflecting "elle" and I'm just wrong?
"y" is an adverb (no gender, no number) and means "there".
The verb "aller" needs a hint of a destination like "there": "y aller" or a starting point, like "from here": "s'en aller" (reflexive).
"Être" verbs like "aller" always agree with the subject, so it does have to be "elle y est allée."
I understand what you mean, but it would be better to refer to the gender of nouns or pronouns as masculine and feminine. The terms "male" and "female" are a biological reference, i.e., sexuality.
She went there. Who went? She went. So "went" agrees with "she" because Aller takes Etre. for verbs which use Avoir, no agreement.
Why not "She has gone there yesterday"? I'd say it could be used in English, if it led to a present result. Am I mistaken?
Yesterday indicates that the whole action took place in the past. "She went there yesterday" means that she left and also arrived yesterday.
"She has left yesterday" is not possible in English. It has nothing to do with whether she's arrived somewhere. "She has left" means the leaving occurred any time up to the present moment, and by adding a limit to how recent it was ("yesterday") you make it impossible to use the perfect tense.
Don't use present perfect tense (e.g., has gone) when there is a clear reference to a time period, e.g., yesterday, Friday, last month, etc.
I'm trying to get my head around the placing of words in French (particularly with negatives, but also here). Why does the "y" come before "est"? It sounds very unnatural for my English-speaking ears, and the only logical explanation I can think of is that "aller" and its auxiliary verb must be kept together as a unit - am I somewhere near the truth on that?
If you remember "il y a" and "il n'y a pas", you will see that "elle y va / elle n'y va pas / elle y est / elle n'y est pas /elle y est allée / elle n'y est pas allée" all work alike.
Bonus for you: it also works with "en": "elle s'en va, elle ne s'en va pas, elle s'en est allée, elle ne s'en est pas allée". (s'en aller = to go (away).
Thanks - remembering "il y a" will hopefully do the trick! And in the negative, the "pas" always follows the conjugated verb? Grabbing hold of the French grammar finger by finger! ;)
My mouth hurts saying this sentence in French:-( The audio version does not pronounce the "t" in "est". Since it comes before a vowel in "allée" shouldn't the "t" be pronounced?
Yes, the T liaison between "est" and "allée" is necessary but the Text To Speech does not always work properly.
"She went yesterday" should be accepted, since it's far more normal to omit "there" in English even when you would need y in French: "Did she go to the cinema? -- Yes, she went yesterday." And omission of "there" in such contexts has been accepted in every other sentence Duo has given me so far in this lesson.
I agree with the comments below - there is I believe no need for "there" in the sentence. "On y va" or "allons-y" mean "let's go" and the "y" is there only to provide the object for the transitive verb.