Learn Arabic (Modern Standard): انكسر الزجاج فجأة.

The basic Form I verb كسر means "to break [something]" (like someone breaking a window). Adding the انـ prefix turns it into the Form VII verb انكسر, which is intransitive. This means the action happens to the subject itself without mentioning an outside actor, translating perfectly to "the glass broke."

How can I recognize that انكسر is a Form VII past tense verb?

Form VII past tense verbs are built on the pattern infa'ala. You can spot it by the انـ (in-) added right before the three root letters (ك-س-ر). You also know it is past tense because it has no present-tense prefixes (like يـ or تـ). Since الزجاج (the glass) is a masculine noun, the verb stays in its simplest 3rd-person masculine singular past form.

Since the glass received the breaking, shouldn't الزجاج be the object of the sentence?

Logically it receives the action, but grammatically, no! Because we are using the intransitive Form VII verb انكسر, the word الزجاج is the grammatical subject of the sentence. In Arabic's standard Verb-Subject-Object word order, the subject comes right after the verb, which is why we see انكسر followed immediately by الزجاج.

Why does the word فجأة (suddenly) end with those two little dashes on top?