Learn Hardcore French: Hier, il y avait un orage, donc nous n’y sommes pas allés. - Yesterday there was a storm, so we did not go there.

Il y avait is the imparfait of the impersonal verb il y a, and it describes a background situation or ongoing state (“there was/there were” in a descriptive sense).
By contrast, il y a eu un orage (passé composé) reports a specific, completed event (“there was a storm” as an isolated fact). In context, the imparfait sets the scene and explains why “we didn’t go.” You could use il y a eu, but it sounds like you’re emphasizing the moment the storm began rather than its overall disruptive presence.

What does the pronoun y stand for in nous n’y sommes pas allés, and how do I identify its antecedent?

In this sentence, y replaces a prepositional phrase indicating a place (usually introduced by à, or “there”). Here it refers back to “the place where the storm was” (i.e. “that place,” often understood from context). Whenever you see y with aller, think “we didn’t go there.”

Is the y in il y avait the same as the y in nous n’y sommes pas allés?

No. In il y a/vait, y is part of the impersonal construction “il y a” (“there is/was”) and doesn’t replace anything. In n’y sommes pas allés, y is a true pronoun replacing a locative phrase.

Why is aller conjugated with être in the passé composé and not avoir?

Aller is one of the so-called “Dr & Mrs Vandertramp” (verbs of motion) that take être as their auxiliary in compound tenses. So you say nous sommes allés, not nous avons allé.

Why does the past participle allé become allés in nous n’y sommes pas allés?

When you use être as the auxiliary, the past participle must agree in gender and number with the subject. Nous here is (at least) masculine plural or mixed, so allé takes an -s: allés.

In nous n’y sommes pas allés, why is y placed before sommes and not next to allés?

French object pronouns generally precede the conjugated verb. In compound tenses, they go immediately before the auxiliary (être or avoir). Thus it’s nous ne y sommes pas allés, never nous ne sommes pas allés y.

How does the negation ne…pas interact with the pronoun y in this sentence?

The order is: ne + pronoun + auxiliary verb + pas + past participle.
So you wrap ne around the pronoun and the auxiliary:
– nous ne y sommes pas (auxiliary)
– allés (past participle)

What is the role of donc in …donc nous n’y sommes pas allés, and do I need a comma before it?

Donc means “so” or “therefore” and introduces the logical consequence of the first clause. A comma before donc is common to signal the pause, but it’s not strictly mandatory. It just helps readability.

Why is the time adverb hier placed at the beginning of the sentence? Can it go elsewhere?

Adverbs of time like hier often appear at the start to set the temporal scene. You could also say Nous n’y sommes pas allés hier or Il y avait hier un orage (though the latter is less idiomatic). Moving hier shifts the emphasis slightly, but all positions are grammatically correct.