Questions & Answers about بنى الملك قصرا ضخما.
Why do قصرا and ضخما both end with an extra alif (ا)?
Because they are in the accusative case! You are currently learning that the second part (the predicate) of a كان sentence takes this case, but direct objects of verbs like بنى (built) also use it. When a word is indefinite (doesn't have 'the') and accusative, it gets an 'an' sound at the end. In spelling, this usually requires adding an extra alif (ا) to hold the double fatha.
Why does the adjective ضخما take the exact same ending as the noun قصرا?
In Arabic, adjectives are strict copycats. They must perfectly match the noun they describe in gender, number, definiteness, and grammatical case. Since the noun قصرا (palace) is an indefinite accusative word with that extra alif, the adjective ضخما (huge) must match its spelling and ending exactly.
The verb بنى (built) ends with what looks like a yaa (ي) but without dots. What is this?
That is a special letter called an 'alif maqsurah' (ى), which literally means 'shortened alif'. Even though it looks like a dotless yaa, it is pronounced exactly like a regular alif (making a long 'aa' sound). You will see this letter very often at the end of Arabic past tense verbs.
Why does the sentence start with the verb بنى (built) instead of the subject الملك (the king)?
While English uses Subject-Verb-Object ('The king built a palace'), the default word order in Arabic is Verb-Subject-Object ('Built the king a palace'). You can say الملك بنى if you want to emphasize the king, but starting with the verb is the most standard and classic way to build an Arabic sentence.