This is the last Grammarology column for 2011, but I’ll be back in January. In the mean time, please, send in your grammar questions. Anything from the trivial to the profound is welcome.
Q: Ian G. wrote:
“Last night a student asked me what the difference was between the following two questions, and which one was best:
1. Have you brushed your teeth? [present perfect]
2. Did you brush your teeth? [past simple]
The context is, for example, a mother asking their child before bedtime. Both versions look acceptable to me and I was at a bit of a loss as to explain the difference.”
A: There are multiple differences, but Ian’s right: both questions are perfectly acceptable.
In many situations present perfect and simple past are interchangeable, but once one has been chosen, certain constraints apply.
Here, I’ll present a fairly traditional view and mention a different analysis in the bonus material at the end.
The choice of present perfect or simple past expresses the way the speaker wants to portray the situation. It’s something like the difference between the ball hit him and he was hit by the ball or between it’s under the book and the book’s sitting on it. In basic cases like  and , both options are available; the speaker can choose freely between them depending on how they view the situation.
In , the asker is choosing to locate the question in a range of time that starts in the past and comes up to the present as in figure 1.
Figure 1. The present perfect showing the situation located at an indefinite time before the present.
In , the asker is choosing to conceive of the brushing (or not) as an act located at a point in past time isolated from the present as in figure 2.
Figure 2. The simple past showing the situation located in the past.
The grammatical consequences for this are not obvious in the above questions, but consider which adverbs might work in conjunction with the sentences. For example, yet and already would work well with  but would be unacceptable to many native speakers with . Conversely, at 8:00 works with  but is unacceptable for  in the situation described. In other words, although the speaker may choose to view the situation in one way or another, once the decision is made, then a number of other options close: you cannot, for example, choose both at 8:00 and the present perfect.
There are also interesting dialectal and genre differences. It’s sometimes thought that the present perfect is more common in the UK and less common in North America, but things aren’t all that clear. It’s very difficult to find comparable data about the two varieties of English, especially when it comes to spoken English. Perhaps the best comparison would be between the British National Corpus and the Corpus of Current American English, keeping in mind that the BNC is from the 1980s to 1993 and the COCA is from 1990 to 2011. I’ve searched for have + past participle1 in the two subsections of these corpora that are most similar in composition: fiction and newspapers. The results are presented in table 1.
Table 1. Comparison of the frequency per million words of have + past participle across dialects and genres per million words.
There is, however, some evidence that use of the present perfect is declining over time, as figure 3 shows. Thus, even these comparisons may not be valid since the BNC ends just when the COCA is getting started. The first row shows the years, the second shows the number of hits, but doesn’t take into account the size of each section. The third row is the important one as it shows the hits per million words.
Figure 3. Frequency per million words of have + past participle in the COCA from 1990 to 2011. (The first number in black is raw hit counts.)
The Cambridge Grammar of the English Language rejects the idea of the present perfect and rather considers English to have two past tenses. The primary one being the traditional past tense or preterite and the secondary one being the perfect tense. Under this analysis, the preterite indicates an action before now, making it deictic while the perfect tense indicates an action before some time established in the discourse, making it non-deictic. When the time established in the discourse is now, then either past tense is available.
The discussion provides a quick overview of the choice between the simple past and the present perfect as used in examples  and . It suggests that teachers need to be very careful about using tests and exercises designed to practice this distinction. In many cases either option would be acceptable, despite what the answer key may say.
1 This is only a rough measure since it will also catch instances of the infinitive perfect (e.g., they seem to have made no progress), and it ignores any cases of the present perfect where something intervenes between have and the past participle (e.g., I have never seen it), when have is contracted (e.g., I’ve seen it), and a variety of other cases. It should, however, produce roughly the same rate of error across genres and frequencies and so should give a good idea of the relative differences between UK and US dialects and fiction and newspaper genres.
Grammarology 2.0 aims to take the discussion of grammar up a notch or two (your mileage may vary) and should appear about once every two weeks. It’s written by Brett Reynolds, who teaches EAP at Humber College and is just about as nerdy about grammar as you can get. He’s is always happy to take questions, which you can send to firstname.lastname@example.org.
Once again, my thanks to Marianne Vespry for comments on previous versions!