Learn Hardcore Italian: È raro che la cassiera dimentichi lo sconto alla cassa. - It is rare for the cashier to forget the discount at the register.

Because È raro che is an impersonal expression expressing rarity or subjectivity, and after such expressions Italian requires the subjunctive mood in the dependent clause. The present subjunctive of dimenticare for “lui/lei” is dimentichi, whereas dimentica is the indicative form and would be ungrammatical here.

What role does che play in this sentence? Can it be omitted?

Che is the conjunction linking the main clause (È raro…) to the subordinate clause (la cassiera dimentichi…). You cannot drop it in this structure. If you remove che, you’d have to change the syntax (for example to an infinitive construction like È raro dimenticare…).

Why is la cassiera used instead of just cassiera or una cassiera?

Italian often places the definite article before professions or roles, even in a general sense. Here:

la cassiera refers to the (female) cashier at the counter
una cassiera would imply any cashier out of many
dropping the article entirely sounds ungrammatical in context

Why is lo sconto used rather than il sconto?

Italian definite articles depend on the initial letter(s) of the noun:

Before masculine nouns beginning with s + consonant, z, ps, gn, pn, x or y, you use lo instead of il.
Since sconto starts with sc, the correct form is lo sconto.

What does alla cassa mean, and why is it alla instead of in or just a?

Alla is the contraction of a + la, meaning “to the” or “at the.”

Cassa here means the cash register or checkout desk.
Alla cassa therefore means “at the checkout.”
Using in would suggest being inside something, and a alone would lack the definite article.

Why is the subject la cassiera repeated, given that Italian often drops subjects?

In Italian you can drop subject pronouns because the verb ending usually indicates the subject. However, the present subjunctive form dimentichi is identical for io, tu and lui/lei, so without la cassiera the clause would be ambiguous. Naming the subject clarifies who is doing the action.

Could we rephrase this sentence using an infinitive, like È raro dimenticare lo sconto alla cassa? What changes?

Yes, È raro dimenticare… with an infinitive is grammatically correct but becomes a general impersonal statement (“it’s rare to forget the discount at checkout”). It doesn’t specify who might forget. The subjunctive version (È raro che la cassiera dimentichi…) highlights a particular subject and expresses a direct judgment about her action.

What’s the difference between È raro che la cassiera dimentichi lo sconto alla cassa and La cassiera raramente dimentica lo sconto alla cassa?

The first uses an impersonal expression + subjunctive to comment on the event’s rarity from a general perspective.
The second is a straightforward active sentence with the adverb raramente and the indicative mood.
Both convey the same basic idea, but the impersonal-subjunctive form can sound more formal or evaluative, while the active-indicative form is more direct.

Why does È carry an accent, and how is it pronounced?

The grave accent on È marks it as the verb essere (“to be”, third person singular) and distinguishes it from e (“and”). It’s pronounced [ɛ], an open-mid front vowel (similar to the e in English “bet”).