The object مَفعُولٌ بهِ maf’ūl bihi is the thing to which the action is done. It is always in the state of Nasb. i.e the accusative case.
اِشتَرَيتُ خُبزًا
I bought bread
أَكَلَ العِنَبَ
He ate the grapes
كَسَّرتُ القلَمَ
I broke the pen
The object can also occur in the form of attached pronouns,
ضَرَ بَــهُ أَحمَد
Ahmed hit him
رَأَيتُـهُم فِي السُّوقِ
I saw them in the market
أَعرِفُــهُ جَيِّدًا
I know him well
The attached pronouns in these examples don’t end with the fatḥa even though they are in the accusative case because they have fixed word endings (mabni).