First, I'll explain the grammatical difference between the 2 words : In Arabic there are 3 cases (Nominative, Accusative, and Genetive) Nouns and adjectives ending with ـــٌ (for indefinite) or ـــُ (for definite) are in the nominative case. Nouns and adjectives ending with ـــً (for indefinite) or ـــَ (for definite) are in the accusative case. Nouns and adjectives ending with ـــٍ (for indefinite) or ـــِ (for definite) are in the genetive case. So, here we have مَلِكٌ in the nominative case, and مَلِكًا in the accusative case. Examples : في اِسْبانِيا, حَكَمَ مَلِكٌ لِمُدَّةِ عَشْرِ سَنَواتٍ. In Spain, a king ruled for 10 years أَصْبَحْتُ مَلِكًا شُجاعًا I became a brave king. I hope this will help you. Best of luck.

(ملك =وَكَانَ وَرَاءَهُمْ مَلِكٌ يَأْخُذُ كُلَّ سَفِينَةٍ غَصْبًا ﴿٧٩ الكهف (ملكا =إِذْ قَالُوا لِنَبِيٍّ لَهُمُ ابْعَثْ لَنَا مَلِكًا نُقَاتِلْ فِي سَبِيلِ اللَّهِ ﴿246 البقرة I hope its clear

النجاشيٌّ ملكٌ عادلٌ كان النجاشيُ ملكاََ عادلاََ

كان محمد ملكاً على السودان = mohamed was the king of SUDAN ان محمد ملكٌ على السودان =mohamed is the king of SUDAN

الله ملك السموات و الارض اصبح هذا المنزل ملكا لى