"Those are they from whom We accept," meaning: Allah accepts from them "the best of what they did."
"Among the companions of the Paradise," meaning: with the companions of the Paradise. "A promise of truth that they were promised" in the world.
Muhammad said: "A promise of truth" is in the accusative case as a corroborative verbal noun (masdar mu'akkid) for what preceded it.
﴿أُولَئِكَ الَّذين يُتقبل عَنْهُم﴾ أَي: يتَقَبَّل اللَّه مِنْهُم ﴿أَحْسَنَ مَا عمِلُوا﴾.
﴿فِي أَصْحَاب الْجنَّة﴾ مَعَ أَصْحَاب الْجنَّة ﴿وَعْدَ الصِّدْقِ الَّذِي كَانُوا يوعدون﴾ فِي الدُّنْيَا.
قَالَ محمدٌ: ﴿وَعْدَ الصدْق﴾ منصوبٌ مصدر مُؤَكد لما قبله.
