The importance of learning Usool al-Fiqh for the one who wants to learn fiqh
Question

                If a seeker of knowledge wants to learn fiqh, can he do without learning usool al-fiqh (the bases of fiqh)?
Praise be to Allah. 
          Shaykh Muhammad ibn Saalih al-‘Uthaymeen 
          (may Allaah have mercy on him) was asked this question and he replied 
          as follows: 
 
          If a seeker of knowledge wants to be a scholar of fiqh 
          then he must combine fiqh and usool al-fiqh in order to be erudite and 
          a specialist in that field. Otherwise you may know fiqh without knowing 
          its usool (bases), but you cannot know usool al-fiqh and become a faqeeh 
          without knowing fiqh. In other words, it is not possible for the faqeeh 
          to do without usool al-fiqh or for the usooli (the one who knows usool 
          al-fiqh) to do without fiqh if he wants to learn fiqh. Hence the scholars 
          of usool differed as to whether it is better for the seeker of knowledge 
          to start with usool al-fiqh so that he can build his knowledge of fiqh 
          on that basis, or to start with fiqh because that is needed, i.e., people 
          need to know that with regard to their worship and dealings with others 
          even before they learn usool al-fiqh. The second is more correct and 
          is that which is usually followed.