An existing translation of as-Sa'dee's work titled (رسالة لطيفة جامعة في أصول الفقه المهمة) "A Brief, Concise Treatise Regarding Important Principles of Fiqh" has been used as a basis for these articles with corrections to numerous errors in the translation, improvements and amendments to make the material easier to read and comprehend. This translation was received and published 14 years ago on the Salafi Publications website and is also widely distributed on the web.
Chapter Five: The Rule Concerning the Commands in the Book and the Sunnah
الأصل في أوامر الكتاب والسنة : أنها للوجوب ، إلا إذا دل الدليل على الاستحباب أو الإباحة ، والأصل في النواهي : أنها للتحريم ، إلا إذا دل الدليل على الكراهة. والأصل في الكلام : الحقيقة فلا يعدل به إلى المجاز ? إن قلنا به ? إلا إذا تعذرت الحقيقة ، والحقيقة ثلاثة : شرعية ، ولغوية ، وعرفية. فما حكم به الشارع وحَدَّهُ : وجب الرجوع فيه إلى الحد الشرعي ، وما حكم به ولم يحدَّه اكتفاءً بظهور معناه اللغوي : وجب الرجوع فيه إلى اللغة ، وما لم يكن له حد في الشرع ولا في اللغة : رجع فيه إلى عادة الناس وعرفهم ، وقد يصرح الشارع بإرجاع هذه الأمور إلى العرف ، كالأمر بالمعروف ، والمعاشرة بالمعروف ونحوهما ، فاحفظ هذه الأصول التي يضطر إليها الفقيه في كل تصرفاته الفقيه .
The asl (fundamental principle) concerning commands in the Book and the Sunnah is that they are indicative of a wujoob (obligation) unless there is an evidence to indicate it being mustahabb (recommended) or mubaah (permissible). The foundation concerning prohibitions is that they are indicative of tahreem (forbiddance) unless there is an evidence to indicate it being makrooh (hated).
The asl governing kalaam (speech) is that it is to be taken upon its haqeeqah (reality). So it is not turned away from (its reality) to its majaaz (figurative meaning) - if we affirm (the presence of majaaz) - except when it not possible to employ it upon its haqeeqah (reality).
Al-haqaa'iq (the realities intended through words) are of three types:  shar'iyyah (that which is defined by the Sharee?ah),  lughawiyyah (that which is defined by language) and  'urfiyyah (that which is defined by customary usage).
So whatever ruling the Shaari' (Lawgiver) has judged with and defined, then it is obligatory to return it to the Sharee'ah definition. However, what the Lawgiver has ruled, but not defined, sufficing by the apparentness of its linguistic meaning, then it is obligatory to return it to its linguistic meaning. But whatever has not been defined, neither in the Sharee'ah, nor in the language; then it is obligatory to refer it back to the habit of the people and their customary useage (understanding). The Lawgiver may clearly specify to return these matters to 'urf (customary usage); such as (what counts as) commanding the good, living well with one's wife, and other similar matters. 
So memorize these usool concerning which the faqeeh (jurist) stands in need of in all his dealings in fiqh-related matters.
 To illustrate with examples (الصلاة) has a linguistic meaning but it is not the meaning intended by its usage in the texts, since the texts have clearly defined what is meant by (الصلاة), which is a collection of words, postures, actions, movements, feelings and inward beliefs that collectively comprise an act of worship. The same with (الإيمان) and what is like that. In other cases, words are not given additional definitions through the texts and so they are taken upon their linguistic meanings known and established in the language. And in yet other cases the customs and habits of the people would define the reality intended by words and phrases. Hence, "living well with one's wife" would indicate such material, financial, emotional affairs that would vary from land to land and time to time, and thus what counts as "living well with one's wife" would be returned back to what is customary and habitual with the people of a given land.