وَلاَ تَدْعُ مَعَ اللَّهِ إِلَهًا اخَرَ لاَ إِلَهَ إِلاَّ هُوَ
...
And invoke not any other god along with Allah, there is no God but Him.
means, it is not appropriate to worship anything or anybody except Him, and divinity does not befit any except His glory.
...
كُلُّ شَيْءٍ هَالِكٌ إِلاَّ وَجْهَهُ
...
Everything will perish save His Face.
Here Allah is telling us that He is Eternal, Ever Lasting, Ever Living, Self-Sustaining, Who, although His creation dies, He will never die, as He says:
كُلُّ مَنْ عَلَيْهَا فَانٍ
وَيَبْقَى وَجْهُ رَبِّكَ ذُو الْجَلْـلِ وَالاِكْرَامِ
Whatsoever is on it will perish. And the Face of your Lord full of majesty and honor will remain forever. (55:26-27)
Allah used the word "Face" to refer to Himself, as He says here:
كُلُّ شَيْءٍ هَالِكٌ إِلاَّ وَجْهَهُ
Everything will perish save His Face.
meaning, everything except Him.
It was reported in the Sahih via Abu Salamah that Abu Hurayrah said,
"The Messenger of Allah said:
أَصْدَقُ كَلِمَةٍ قَالَهَا الشَّاعِرُ لَبِيدُ أَلاَ كُلُّ شَيْءٍ مَا خَلَ اللهَ بَاطِلُ
The truest word of a poet was the saying of Labid - indeed everything except Allah is false.
...
لَهُ الْحُكْمُ
...
His is the decision,
means, dominion and control, and there is none who can reverse His judgement or decision.
...
وَإِلَيْهِ تُرْجَعُونَ
and to Him you shall be returned.
means, on the Day when you will be brought back, and He will reward or punish you according to your deeds: if they are good, then you will be rewarded, and if they are bad, then you will be punished.
This is the end of the Tafsir of Surah Al-Qasas. All praise and thanks are due to Allah.