“Such as” introduces an example, “like” an example or a similarity, and “as” an example, similarity or role
THE CONFUSING NATURE OF “As”, “Like” AND “Such As”
The uses of as, like and such as are very similar. In some languages, indeed, all three are sometimes translated by the same word. Explanations of the difference between as and like are easily found in mainstream grammar books, but mentions at the same time of such as are rare. My hope here is to offer a clear explanation of the more confusing aspects of these three expressions.
Part of the problem with investigating how the three expressions differ is that grammar is involved as much as meaning. There are both preposition uses to consider and conjunction ones.
PREPOSITION USES 1 (Adjectival)
Prepositions usually need to be used in sentences with a noun, often called their “object”, that is usually positioned straight after them (see 84. Seven Things to Know about Prepositions). The two together – normally called a preposition phrase – may act like an adverb, saying something about a verb or the sentence as a whole, or like an adjective, saying something about another noun, usually placed just in front. Preposition phrases starting with as, such as and like are very able to act like adjectives. In the following examples, the noun they go with is prisons
(a) Prisons as rehabilitation centres are a failure.
(b) Prisons, such as/like Alcatraz, instil fear in most people.
(c) Prisons such as/like Alcatraz instil fear in most people.
(d) Prisons, like hospitals, instil fear in most people.
(e) Prisons like hospitals instil fear in most people.
The meanings of as, such as and like used like this are as follows:
As means “in the role of” or even “with the purpose of”. It suggests that the noun after it is naming a special use of the noun before.
Such as introduces one or more examples, just like for example (see 1. Simple Example-Giving, 54. Sentence Lists 1: Incidental and 74. Sentence Lists 3: Bullet Points). It thus indicates that the idea mentioned after it (Alcatraz) is part of a larger group shown by the noun before (prisons). The noun before must, as a result of this, be either plural or a generalizing singular (e.g. a prison).
Such as phrases may go inside two commas, as in (b), or not, as in (c). The difference is similar to that made by using or not using commas with who or which or that (see 34. Relative Pronouns and Commas and 77. Apposition). In other words, sentence (b) is about all prisons, while (c) is about only some (the Alcatraz kind).
Like can be used in much the same way as such as to introduce an example. However, it is probably more used like this in spoken than written English.
Like can also be used both with and without commas to show a similarity, as in (d) and (e). The use with commas is, I think, actually adverbial rather than adjectival, because the like phrase does not have to follow a noun – it can be right at the start of the sentence with a comma after it. For more about this use, see 56. Comparing with “Like” and Unlike” and 149. Saying How Things are Similar. On the other hand, like without commas, as in (e), is adjectival. It indicates the idea of “some”, just as in the exemplifying use. One other point to note is that similarity-showing like cannot be used after the same (see 87. “Same As” versus “Same That”).
You can tell whether a like phrase is giving an example or a similarity by comparing the noun after it with the noun before. If the noun after like means something that is part of what the noun before it means (in the way that Alcatraz means part of the idea of prisons), then an example is being given; whereas if the noun after like does not mean part of what the preceding noun means (in the way that hospitals is not part of what prisons means), then a comparison is being made.
In addition, the noun placed before the comparing use of like does not have to have plural or general meaning. Here is a non-general one:
(f) The new virus produced an illness like influenza.
PREPOSITION USES 2 (Adverbial)
In the examples above, the prepositional as, such as and like all have a noun immediately before them, which they and their own noun describe rather as adjectives do. However, English prepositions can also be used in another way – not describing a preceding noun – and this leads to some slightly different uses of as and like (but not of such as, which can only be used in the way shown above). Consider these examples:
(g) The new circus recruits perform as clowns.
(h) The new circus recruits perform like clowns.
Here the word before each preposition (perform) is a verb. Now the prepositions and their partner nouns are acting like adverbs, saying how the action of the verb happens. However, their meanings are still more or less the same: as means “in the role of” while like means “similarly to”. Thus as says the recruits were clowns, while like says they were not, but just resembled them (see 149. Saying How Things are Similar).
This adverb-like use of as is easily confused with the complement use in sentences such as the following:
(i) Grammar can be described as a challenge.
In both cases, the noun after as stands for the same person or idea as the sentence subject − clowns = recruits in (g); a challenge = grammar in (i). The difference is perhaps that in the complement use as does not mean “in the role of”. More about the complement use of “as” is in the post 92. Complement-Showing “As”.
As and like (but not such as) can also be conjunctions, in other words used with a verb in addition to a noun after them (see 174. Eight Things to Know about Conjunctions). When as is a conjunction it has a variety of well-known uses: introducing a reason, like “because” (see 61. “Since” versus “Because”); or showing that an event is happening alongside another one, like “while”; or introducing the name of a data source (see 104. Naming Data Sources with “As”). Here, however, I wish to consider a conjunction use closer in meaning to the preposition one, like this:
(j) Adjectives can follow BE as/like nouns can.
The verb after this conjunction use of as/like is usually a stand-alone auxiliary – here can – understood as an abbreviated repetition of the main verb – can follow (see 36. Words Left Out to Avoid Repetition). The meaning of like is unchanged (i.e. making a comparison), but that of as is not, since it now means the same as like, and hence indicates that the nouns before and after refer to two similar things, not one thing with two names.
The fact that the conjunction as can mean practically the same thing as the preposition like allows us to paraphrase one as the other. Sentence (h) above, for example, can be paraphrased with as like this:
(k) The new circus recruits perform as clowns do.
This compares new recruits with clowns, just as (h) does. The important thing to remember here is that without the do there is no comparison, only the meaning of “in the role of”.
2. A Special Exemplifying Use Of “As”
Having indicated that as cannot generally be used for giving an example, I have to mention one important exception. This is in sentences like (l):
(l) The letter “u” sometimes has to be pronounced like the letter “e”, as in “bury”
The key requirement for this use is a preposition (commonly in) just before the example. There could also be a verb (e.g. it is) before the preposition, reflecting the fact that as is a conjunction, but that is often left “understood” as in (l) (see 36. Words Left Out to Avoid Repetition). The exact example in (l) is the letter “u” in “bury” and the name of the general class to which it belongs is the letter “u” … pronounced like the letter “e”.