A. It is important to note that not every jaarr wa majrur or tharf construction can be classified as a shibhu jumlah. It is only when the muta'allaq ( that which the jaarr wa majrur or tharf is connected to) is dropped, out of necessity.
e.g. زيدٌ اسْتَقَرَّ أمَامَ البَيت or زيدٌ مَوجُودٌ في الدَّار
Here the words in red are the muta'allaqs to which a jaarr wa majrur (or tharf) must necessarily be connected to. The muta'allaq has been written above but is dropped because to mention the muta'allaq would be redundant.
The actual sentences should read:
e.g. زيدٌ أمَامَ البَيت and زيدٌ في الدَّار
In the case when the muta'allaq is mentioned (whether necessarily or optionally) the jaarr wa majrur and tharf are not technically called the shibhu jumlah
e.g. زيدٌ ذَاهِبٌ إِلَى الْمَدْرَسَةِ
Here the muta'allaq ذَاهِبٌ (highlighted in red) - to which the jaarr wa majrur إِلَى الْمَدْرَسَةِ is connected - must be mentioned in the sentence and cannot be done without otherwise the sentence won't make sense. So you can't just drop ذَاهِبٌ and say زَيْدٌ إِلَى الْمَدْرَسَةِ because this won't make sense unlike the first 2 sentences above.
So the main difference between when and when not to consider a jaarr wa majrur or tharf as a shibhu jumlah has to do with whether or not the muta'allaq is necessarily dropped. If the muta'allaq is necessarily dropped as in زيدٌ مَوجُودٌ في الدَّار , then the jaarr wa majrur or tharf is a shibh jumlah. If the muta'allaq is not dropped as in زَيْدٌ ذَاهِبٌ إِلَى الْمَدْرَسَةِ then the jaarr wa majrur or tharf is not a shibh jumlah.
It should be noted that there are some people who consider any jaarr wa majrur or tharf construction as a shibhu jumlah. This however is not technically correct. A Shibhu jumlah is a specific kind of jaarr wa majrur or tharf construction - the kind in which the muta'allaq must be dropped out of necessity.
The following are the places in which the shibh jumlah are often encountered:
(1) position of khabar e.g. زيدٌ اسْتَقَرَّ في الدَّارِ
(2) position of haal e.g. رَأَيْتُ زَيْدًا يَسْتَقِرَُّ فِي دَارِهِ
(3) position of sifah e.g. رَأَيْتُ رَجُلاً يَسْتَقِرَُّ فِي دَارِهِ
(4) position of silatul mowsul e.g. رَأَيْتُ الرَّجُلَ الَّذِيْ يَسْتَقِرَُّ فِي الدَّار
Now, in each of these cases the jaarr wa majrur في الدَّار or فِي دَارِهِ is a shibhu jumlah, and can be said to be the khabar, haal, sifah, or silah mawsul in place of the muta'allaq that has been dropped. Alternatively, one can still consider the dropped muta'allaq to be the khabar, haal, sifah, or silah mawsul, and the jaarr wa majrur (muta'alliq) connected to it. You will find both these alternatives being used when doing i'raab or tarkib of sentences.
By Nabeel Alkhalidy