What are the rules of copular "to be" tense in pseudo-cleft sentences?

Question

What are the rules for the tense usage of copular "to be" in pseudo-cleft sentences? I have several examples:

1. What I didn't like was the end of the movie. --> was
2. What changed his mind was a book he'd read. --> was
3. What I have done is write a letter to the editor. --> is
4. What he did is unforgivable. --> is
5. What drove us crazy was the noise. --> was (this one is from Parrot's Grammar for English Language Teachers)
6. Why we came late is because we had to finish the work ourselves. -->is (also from the above-mentioned book)
7. What I wanted to say is that you're stupid. --> is
8. What Mary bought was a first edition. --> is

I cannot seem to find any explanation to the usage of "is" vs. "was". The only mention of this that I managed to find is in the F.R.Higgins's "The pseudo-cleft construction in English" (2015, Chapter 6, part 2.1 - Tense Harmony):

That is, there are two tenses, past versus non-past, and the copula may only appear as is or was, regardless of what kind of verb form appears in the what-clause

....in pseudo-cleft sen­tences the tense of the copula must agree with the tense of the verb in the clause.

and also:

Somehow the whole matter of tense harmony is related to the present or past relevance of the subject and the focal item.

But the Parrot's example (#6 in my sentences) seems to contradict this idea:

Why we came late is because we had to finish the work ourselves. "we came late" == past, "we had to finish" == past, but "is" (not "was") is used as a copula.

Is there an explanation or a rule for this concept? Thank you!

Answer 1

Notice how the pseudo-cleft clause is merely a noun clause that serves as the subject of the copular linking verb "to be," so what tense you'd use is only governed by what you wish to convey, for example:

• if you wish to convey how things went or were at the time, including how you felt at the time, then you would use a past tense that is in tense agreement with the past tense in the pseudo-cleft clause (e.g., "What I didn't like was the end...," "What changed his mind was a book...," "What he did was unforgivable," etc.),

• if you wish to convey how things go now or how things are now, possibly emphasizing that present condition or that the result conveyed by the copular "to be" exists in the the present or remains presently, then you'd use a present tense (e.g., "What I didn't like is the end...," What changed his mind is a book..., "What he did is unforgivable," etc., i.e., it's made clear that the end is still what you don't like, his mind remains unchanged or that book still exists and is maybe something you're about to talk about presently, and nothing has interceded since he did what he did to make what he did forgivable such that it is as unforgivable now as it was then, or maybe it was forgivable then but is now unforgivable for some reason, a change in social climate since then, for instance).

As for the third example, since the pseudo-cleft clause uses the present perfect tense, not the past tense like all the others, using the preterit tense "was" for the copular "to be" that links the pseudo-cleft clause to the sentence's predicate would be anachronistic, even a temporal paradox of tense disagreement.

For there to be tense agreement, a present tense, either present simple or present perfect, would have to be used for the copular "to be" linking the pseudo-cleft clause (the subject) to the predicate for the sentence to make sense, for the action in the pseudo-cleft clause not to occur after when it's said to occur with the copular "to be" that links it to the predicate.

The only time machine in grammar is pluperfect. Other than pluperfect, time moves in only one direction: forward. The other examples make sense because time is directionally moving forward, but in the third example, time doesn't move forward. Instead, it directionally jumps backwards with "was" to convey "write letter to the editor" somehow occurs before "what I have done" when "what I have done" and "write a letter to the editor" are being established by the copular "to be" as one and the same-- a grammatical paradox, to be sure.

Therefore, the only possibilities for conjugating the copular "to be" in the third example would be present simple tense "is" or present perfect tense "has been" (i.e., What I have done is write a letter to the editor" or "What I have done has been (to) write a letter to the editor"), depending on what you wish to convey.

Answer 2

What I didn't like [at that time] was the end of the movie. --> was

What changed his mind [at that time] was a book he'd read. --> was

What I have done [now, currently, at this moment] is write a letter to the editor. --> is

What he did is [still, currently, at this moment] unforgivable. --> is

What drove us crazy [at that time] was the noise. --> was

etc.

They have time frames and the tense of "to be" reflects this.

EDIT

It should be understood that "What" = "The thing(s) that"

There are the following possibilities

1. It happened in the past and had its effect only in the past

What he did/had done was/had been a disaster.

2. It happened in the past and had its effect the past and present

What he has done has been a disaster.

3. It happened in the past and had its effect only in the present.

What he did/had done is a disaster.

4. It happens in the present and had its effect in the present.

What he does is a disaster.

5. It happens in the present and will have its effect in the future.

What he does/is doing will be a disaster.