A relative pronoun ism mawṣūl (اسم موصول) introduces a relative clause, which is a subordinate clause. The dependency of a relative clause on a relative pronoun is known as ṣilat l-mawṣūl (صلة الموصول) in traditional Arabic grammar. Verse (103:3) shown below contains a relative pronoun which is followed by a relative clause (صلة):
Fig 1. Relative pronoun and relative clause in verse (103:3).
In general, the Arabic word ṣilat (صلة) means relation. When used to relate words syntactically, the grammatical meaning is of a relative or subordinate clause. As well as a relative pronoun, a subordinating conjunction (حرف مصدري) may be used to a introduce a subordinate clause. The most common such particle is an (أَن) which is usually translated as "that". Verse (96:7) shown below contains a subordinate clause introduced by a subordinating conjunction:
he sees himself
Fig 2. Subordinating conjunction and subordinate clause in verse (96:7).
A subordinate clause may also be introduced by the prefixed lām of purpose (لام التعليل). The subordinating conjunction an ("that") is implied in this construction, as illustrated by verse (72:17) in sūrat l-jin:
That We might test them
surely We (would) have given them to drink
Fig 3. Prefixed lām of purpose and subordinate clause in verse (72:17).
Subordinate Clauses and the Subjunctive Mood
If a subordinating conjunction or purpose particle introduces a subordinate clause that is headed by an imperfect verb, then the verb will usually be found in the subjunctive mood manṣūb (منصوب). There are exceptions to this rule, such as if the verb forms part of a negative expression. Another exception is if the subordinating conjunction law (لو) introduces the subordinate clause, since this particle does not take the subjunctive mood.