1) Usool ul-Fiqh, as we have mentioned, represents the principles (Qawa’id) that the Mujtahid follows in the Istinbat (extraction) of the practical Shari’ah rulings from their detailed evidences like the Qaa’idah (principle):
‘Working with two evidences is better (more proper) than neglecting one of them’.
This is whilst Fiqh is the ‘Ilm (knowledge) of the practical Ahkam Shari’ah taken from its detailed evidences like: Khamr is Haraam, Al-Jihaad is Fard. Tahaarah is a Shart (condition) from the conditions of the validity of the Salaah and like Haid (menses) is A Maani’ (prevention) for the performance of the Salaah.
2) Usool ul-Fiqh includes studies in relation to the language and its principles because these principles are necessary for Ijtihad. This is like: The studies about the Haqeeqah (literal) and Majaaz (metaphorical), the meanings of the Huroof Al-Jarr (prepositions), the Huroof Ash-Shart, and the general Alfaazh (expressions) amongst other matters.
This is whilst Fiqh is restricted to the explanation of the Hukm Shar’i that is a requirement of the Mukallaf like: The Salaah is Fard, ‘Umrah is Sunnah (Mandoob) and Riba is Haraam.
3) Usool ul-Fiqh examines the Ijmaaliy Shar’i Daleel like the Qur’an, Sunnah, Qiyas and Ijmaa’ in respect to establishing that it has come from Allah and then it examines what came within it in terms of principles and forms like: The Khusoos and the ‘Umoom (specific and general), the Mutlaq and the Muqayyad (unrestricted and restricted), Al-Mujmal (general/un-detailed) and Al-Bayaan (explanation) and An-Naskh (abrogation).
This is whilst Fiqh examines the Daleel Al-Juz’iy (partial evidence) that guides to or indicates a specific Hukm for a matter or an action for the purpose of the Mukallaf abiding by it. So for example it will examine the Ayah:
وَقَاتِلُوا الْمُشْرِكِينَ كَافَّةً كَمَا يُقَاتِلُونَكُمْ كَافَّةً
“And fight against the disbelievers collectively as they fight against you collectively”
[TMQ At-Taubah 36]
And the Ayah:
وَأَحَلَّ اللَّـهُ الْبَيْعَ وَحَرَّمَ الرِّبَا
“And Allah has made trade Halaal and made Riba (usury) Haraam” [TMQ Al-Baqarah 275]
And the Hadith:
‘If two Khalifahs are given the Bai’ah then kill the latter of the two’ [Muslim]
And the Ijmaa As-Sahabah (ra) in respect to delaying the burial of the Messenger of Allah (saw) until after they had pledged allegiance to a Khalifah from amongst them.
That is done in order for the Hukm As-Shar’i to be taken from each Daleel (evidence).
4) The aim of Usool ul-Fiqh is the application of its principles upon the detailed evidences to arrive to the manner of how the practical Ahkam Shari’ah are extracted.
This is whilst the aim of Fiqh is to explain and apply the Ahkam Shari’ah upon the actions of the people and their speech and it is to make every Mukallaf aware of the commands of Allah (swt) and what He has forbidden, so that the Mukallaf can adhere to it in his actions.