“The one(s)” is a pronoun, but some nouns cannot be replaced by it
GENERAL USES OF “one(s)”
The word one is sometimes used as a pronoun (see 28. Pronoun Errors). As such, it has no following noun and it takes some of its meaning from another noun (or noun phrase) that is easy to identify, usually from a previous mention, like this:
(a) Some national foods are world-famous. One is pasta.
(b) Various reasons are usually given, but one is not convincing.
(c) The best-supported team is one in the North-West.
In these examples, one means respectively “one world-famous national food”, “one usual reason”, and “one team”.
The meaning that is always present in the word one is non-uniqueness – that the single idea being mentioned is not the only one in existence but belongs to a wider group of similar ideas: world-famous national foods in (a), usual reasons in (b), and teams in the North-West in (c). A similar meaning is present in the concept of exemplification (see 1. Simple Example-Giving), though only (a) above could be understood as exemplifying – the others fail to meet other conditions for exemplification.
We can also use a plural form ones, like this:
(d) The balls were of many colours but subjects tended to choose red ones.
Again, ones suggests that additional, unmentioned things of a similar kind exist, here other red balls plus balls of other colours. It is normal when using ones to add some describing language, for example an adjective in front (red ones) or a relative clause after (ones that were red). The technical name for such added description is “modification”, more about which can be read in the post 15. Half-Read Sentences.
Another noun-like feature of both one and ones is their ability to have the or a(n) before them. However, this is only possible when there is also modification – we cannot say *a one or *the one, only a/the red one or a/the one in the North-West. We choose between a and the in the same way as we do with countable nouns.
In this post I wish to concentrate on the use of the with one and ones.
WHEN TO USE “the one(s)”
The effect of using the with one(s) is to combine the basic meaning of “not all” with a further meaning of “all”. In sentence (d), for example, using the red ones would still mean not all of the balls (since it would imply the existence of non-red balls), but it would also mean all of the red balls, and not just some of them as red ones would. Here is another example of the use with the, this time involving singular one:
(e) Her time was the fastest one ever achieved.
Here one still conveys the meaning of “not all”: there have been other “times achieved”. However, the one refers to all of the fastest times ever achieved.
WHEN NOT TO USE “the one(s)”
There are some situations where the meaning of the one(s) needs to be expressed, but different words have to be used. It is here where grammar errors are likely to occur. There are three main situations of this kind.
1. Referring to an Uncountable Noun
We cannot use one(s) or the one(s) to refer to an uncountable noun. What could be used instead of the one in the following examples (uncountable noun underlined)?
(f) Most countries experienced economic growth, but the …. in South Africa was remarkable.
(g) The participants had to choose between the green food and the red … .
The problem in both of these examples could be solved by using a category noun instead of one, e.g. kind or version. Alternatively, sentence (f) – but not (g) – could use the pronoun that in place of both the and the noun. This pronoun is possible provided there are modifying words after it, like … in South Africa in (f). It can replace countable nouns as well as uncountable ones. In sentence (g), where the modifying word (red) comes first, a different alternative to using a category noun still exists: simply leaving that noun out altogether, so that the sentence ends … and the red. This is similar to leaving words out to avoid repetition (see 36. Words Left Out to Avoid Repetition). It is even possible when the earlier noun is countable (i.e. when you could also say the red one(s)).
2. Formal Writing
Formal writing is considered in detail in the post 46. How to Avoid “I”, “We” and “You”. It allows some uses of the one(s) but not others. One allowed use is with a describing word, such as red, between the and one(s); another is when one(s) stands for a person rather than a thing, like this:
(h) Three people will be interviewed and the one who performs best will be selected.
In other cases, however, it is considered more formal to write that/those instead of the one(s), like this:
(i) The company welcomes the comments of employees as well as those (= the ones) of customers.
3. Comparison Sentences
The choice between that/those and the one(s) to represent a countable noun also seems less free in sentences that make a direct comparison, like these:
(j) The unemployment rate in country A is higher than that (= the one) in country B.
(k) The abilities of human beings at birth are very undeveloped compared to those (= the ones) of most animals.
(l) The function of the human heart is similar to that (= the one?) of a pump.
It is usually better to say that/those than the one(s) in such sentences (unless you mean “the person”). Sentence (l) in particular seems strange if the one is used.
This and other points about comparisons are all considered together in the post 82. Pitfalls in Making Comparisons.