A prepositional phrase or an adverb may be a predicate, but it is not the subject. When a nominal sentence begins with one of these phrases, look after it for the subject.
Sentence
Fronted predicate
Subject
فِي الْبَيْتِ رَجُلٌ
فِي الْبَيْتِ
رَجُلٌ
مَعَ الْعُسْرِ يُسْرٌ
مَعَ الْعُسْرِ
يُسْرٌ
فِي الْجِسْمِ مُضْغَةٌ
فِي الْجِسْمِ
مُضْغَةٌ
لَهُمْ عَذَابٌ عَظِيْمٌ
لَهُمْ
عَذَابٌ عَظِيْمٌ