These two kinds of adverbial clauses are not easily kept apart. Sometimes the clause is clearly one of manner, and does not contain or imply any comparison, as in the following sentences: You must explain Barbary to him as best you can. Sometimes, on the other hand, the clause is clearly one of comparison, and does not contain or imply an indication of manner, as in the following sentence: His wife must be a lady and a lady of blood, with as many airs and graces as Mrs Wilkes and the ability to manage врур as well as Mrs Wilkes ordered her own domain.
But there are also sentences where it may be argued, either that the comparison is merely a way of indicating the manner of an action, or that the comparison is the essential point, and the indication of manner merely an accompanying feature.
Since the problem of which view is the correct one, that is, whether the comparison or the indication of manner is the essential point, cannot be solved by objective methods, it is best to say that in such cases the distinction between the two types is neutralised, and that is what makes us treat the two types under a common bending, "clauses of manner and comparison".
The most typical conjunction in such clauses is the conjunction asand indeed, historically speaking, this is its earliest application in the language. The conjunction as is of course also used to introduce clauses of time and of cause, and it is only the context, that is, the lexical meanings of the words, that makes it clear what the function of the clause is. For instance, in the following example it is the meaning of the words make money, repeated as they are, that shows the clause to be a clause of comparison and not a clause of time or cause: With the idea that she was as capable as a man came a sudden rush of pride and a violent longing to prove it, to make money for herself as men made money! It is typical of as-clauses of comparison that the conjunction may have a correlative element in the head clause, which is usually another as. This may be seen in the following example, which is somewhat peculiar: Do you find Bath as agreeable as when I had the honour of making the enquiry before? The when-clause as such is a temporal clause: it indicates the time when an action ("his earlier enquiry") took place. However, being introduced by the conjunction as, which has its correlative, another as, in the main clause, it is at the same time a clause of comparison. It would seem that these two characteristics do not contradict each other but are, as it were, on different levels: the temporal clause occupies a position which might also be occupied by an adverbial modifier of time within a simple sentence, if, for instance, the sentence ran like this: Do you find Bath as agreeable as last year? In that case the phrase as last year would have been a subordinate part expressing comparison, while last year as such would have been an adverbial modifier of time. Such different levels of syntactical analysis do not appear to have received sufficient attention so far.
There may be some argument about the exact status of the as in the head clause. It may be said either that it is an adverb modifying the adjective or adverb which follows it, or that it makes part of a double conjunction as ... as, whose first element is within the head clause, while the second element introduces the subordinate clause. The first view is distinctly preferable, as the idea of an element of a subordinating conjunction coming within the head clause and tending to modify one of its parts is theoretically very doubtful.
Another variant including the conjunction as is the phrase in the same way as (in the same manner as), whose composition and function may be a matter of discussion. It may be taken as a phrase equivalent in function to a conjunction, and thus belonging in its entirety to the subordinate clause. Or else the phrase in the same way as may be viewed as divided between the head clause and the subordinate clause, only as belonging to the subordinate, and in the same way making part of the head clause as an adverbial modifier of manner. There seems to be no valid objective method of setting this question and it remains largely a matter of individual opinion. It may perhaps be argued that some sentences rather incline toward one interpretation, and others toward the other.
Another conjunction used to introduce clauses of comparison is than. It is naturally always associated with the comparative degree of an adjective or adverb in the head clause, as in the sentence: Nobody can appreciate it more than I do. Than-clauses do not seem to offer occasion for any special comment.
Let us now turn to the question of clauses of manner and comparison and adverbial modifiers in a simple sentence.
It is quite clear from the outset that a clause of comparison or manner is used when an action described in the head clause is to be characterised by comparing it to some other action. Adverbial modifiers in a simple sentence give only limited possibilities for this. They can be used to express that sort of idea if the comparison is not, strictly speaking, between the actions themselves but between different subjects performing the same action. This particular kind of comparison may indeed be expressed with the help of the conjunction like, as in the following example: I never see a young, woman in any station conduct herself like you have conducted yourself. (DICKENS, quoted by Poutsma) This usage belongs to low colloquial style.
A similar kind of idea can also be expressed by means of a dependent appendix introduced by the conjunction as. In fact in some cases the difference between a simple sentence with a dependent appendix of this type and a complex sentence with a subordinate clause of comparison appears to be very slight: one may be changed into the other by merely adding or dropping the corresponding form of the verb do or be: He works as efficiently as you (do), He was as excited as she (was), etc. It is therefore natural that sentences without the form of do or be should have been considered as elliptical, with the verb "understood". However, as we have adopted the principle of not admitting ellipsis unless this is strictly necessary, we have chosen to treat those sentences (without do or be) as simple ones with a dependent appendix. So, accordingly, comparing them now with the complex sentences, we may state that the difference in such cases appears to be stylistic rather than anything else. The complex sentences are somewhat more literary in style than the simple ones with the dependent appendix introduced by the conjunction as.
The same considerations apply to the subordinate clauses with the conjunction that and simple sentences with a dependent appendix introduced by the same conjunction: compare I am taller than he (is), He works better than they (do), etc.