Two earlier postings covered these and those, the plurals of this and that, as demonstrative pronouns. Today's covers this and that as singular demonstrative pronouns. (Curiously, Fowler has no entries for them in this or that form.)

Their use in distinguishing time, order, or nearness is straightforward.

This is a pleasant meal.

That was a pleasant meal.

I prefer this to that.

But where this and that are demonstrative pronouns referring to an earlier sentence, matters can become more complicated. Consider this except from a piece in The Economist called 'Time to buy.'

But neither government bond markets nor commodities can in any sense be described as being near a bottom.

In short, this does not look definitely like the kind of low from which very good long-term returns can be made. That may be because the market has a lot further to fall; Morgan Stanley suggests that, if the superbear argument is correct, equities could drop a further 50%. But that will surely require some kind of economic news of the kind seen in the 20^th century.

The present tense (does not look) governs the use of this in the first sentence. And present tense (may be) would indicate the use of This in the second sentence as well, but having already used this in the preceding sentence, the writer switches to That to avoid repetition and possible confusion over the antecedent. Future tense (will surely require) would indicate the use of this in the third sentence too, but distance in time (seen in the 20^th century) indicates that, overriding tense.

Here are a few more from a piece in The Economist on chemical sensors, 'Gas, gas, quick boys.'

In this way the components of the sample separate from one another. Each component is puffed onto the nanotubes, where it sticks to the carbon atoms. This, in turn, causes the conductivity of the nanotubes to change--how much is a characteristic of each gas.

What is needed is a cheap way of detecting such gases and, having raised the alarm, of identifying which gas is involved so that anyone who has succumbed can be treated.

And that is what a team of chemical engineers at the Massachusetts Institute of Technology, led by Michael Strano (pictured on the left), think they have created.

Present tense dictates the use of This in the first pair. It should also dictate the use of this in the second.

Perhaps the writer felt that that would be more demonstrative than this. Or perhaps the writer preferred the word with an abrupt finish over the one with a hiss, a preference I'm beginning to note.

For more on that, view http://www.youtube.com/watch?v=Zo1XFz0kac0.

The demonstrative pronoun these can precede a noun as an adjective: these apples are ripe. Or it can stand alone as a substantive, acting as a noun: these are ripe.

Consider this pair of sentences from a recent Economist:

In a bid to salvage his reputation, Mr Badawi has belatedly started keeping the bold promises of reform that he made on coming to office in 2003. Foremost among these was curbing corruption within the government.

Here, these is the object of the preposition among, acting as a pronoun and standing for promises.

These can often stand alone as the subject of a clause, if it's clear what these refers to (see below). But these works less well as the object of a preposition or of a verb form. I usually switch to the pronoun them, which for me works much better.

'the bold promises of reform that he made on coming to office in 2003. Foremost among them was curbing corruption'

If it's not clear what these or them refers to, I insert the noun, switching these from a substantive to a demonstrative adjective ('Foremost among these promises').

Another usage is as follows:

 The case studies come next. These are straightforward and require mainly copyediting.

My colleagues Meta, Elaine, and Chris would write they, because these implies the existence of those, as though there's another set of book elements that are not straightforward. If there is another set, these would be acceptable, though I'd probably stick with they. Nick has another view (see his comment on this post).

One problem with the standalone substantive is that it usually forces the reader to stop and go back to look for the noun it refers to.

Less of a problem is announcing the noun:

These, then, are the reasons for taking the new threats seriously.

A nice rhetorical flourish.

But why not write those, then, are the reasons? More on this next week.