Learn Arabic (Modern Standard): انكسر مقبض الباب فاستعنا بقفال. - The doorknob broke, so we sought help from a locksmith.

انكسر is a Form VII verb, which makes the action reflexive or passive. While كسر means "he broke [something]", انكسر simply means "[the object] broke" or "was broken". This form is perfectly suited for things that break on their own or when the person who broke it isn't mentioned.

Why does مقبض lack a definite article when it means "the doorknob"?

This is an idafa (possessive construction) meaning "the handle of the door". In an idafa, the first word (مقبض) never takes the definite article الـ. It automatically becomes definite because it belongs to the second word (الباب), which does have the definite article.

How does the verb استعنا break down, and why is its middle vowel missing?

The word starts with the prefix فـ meaning "so" or "then". The verb itself is استعان (to seek help), a Form X verb built on the root for help. Because the root has a weak middle letter (a hollow verb), that long vowel disappears when we attach the "we" suffix ـنا in the past tense. This prevents two unvoweled letters from clashing.

Why is there a بـ attached to the word for locksmith (قفال)?

The verb استعان (to seek help) always pairs with the preposition بـ (with/by means of) to indicate who or what you are getting help from. So, استعنا بقفال literally translates to "we sought help by means of a locksmith".