Строй современного английского языка

Chapter XXXVII
ADVERBIAL CLAUSES
We must start the discussion of adverbial clauses by pointing out that the term "adverbial" should not be taken as an adjective derived from the noun "adverb" (which would make it a morphological term), but as a syntactical term, in the same way that it is used in the phrase "adverbial modifier" denoting a secondary part of the sentence.
With reference to adverbial clauses a question arises that is not always easy to answer, namely: whether they modify some part of the main clause or the main clause as a whole. The answer may prove to be different for different types of adverbial clauses and the question will have to be considered for each type separately. The criteria to be applied in settling this question have, however (at least partly), to be stated in advance.
We will first try out a method that has proved valid, on the whole, for determining whether a clause is an object clause or not. It will serve both for finding whether a clause is an adverbial clause or not, and if it is one, what it modifies. The method consists in dropping the clause in question and finding out what has been lost by dropping it and what part of the main clause has been affected by the omission (it may be the whole of the main clause). If this method does not yield satisfactory results in some particular case we will think of possible other ways of ascertaining the function of the subordinate clause.
The conjunctions introducing adverbial subordinate clauses are numerous and differ from each other in the degree of definiteness of meaning. While some of them have a narrow meaning, so that, seeing the conjunction, we may be certain that the adverbial clause belongs to a certain type (for example, if the conjunction is because, there is no doubt that the adverbial clause is a clause of cause), other conjunctions have so wide a meaning that we cannot determine the type of adverbial clause by having a look at the con-junction alone: thus, the conjunction as may introduce different types of clauses, and so can the conjunction while. With these conjunctions, other words in the sentence prove decisive in determining the type of adverbial clause introduced by the conjunction. ¹