I started this thread to try to fact find and wanted to update it, in case anyone else is interested. It started with just a question about one particular tense, but for me it led to questions about literacy and the effect of grammar and language teaching on that. I am not an expert about any of this, and so if anyone has more knowledge please do comment!
After the last post I started looking more at grammar teaching and it seems that since the 1960s in the UK (situation in UK is quite different from our friends on the continent) there have been two schools of thought about how grammar should be taught, prescriptive (formal/traditional rules) and descriptive (books saying how language is currently used and not saying what is correct or not correct, and this still seems to be a trend in the UK and it appears to have had a disastrous effect on literacy).
An OECD report of 2019 said that levels of literacy in the UK in 55-65 years group is significantly higher than school leavers. Formal grammar stopped being taught to all children at the time 55-65 year olds were at school, and since then teaching has been sparodic until national curriculum changes came in in 2016. Literacy levels in the generation before 55-65 year olds are higher still according to information available - ie this was time when all primary school children were being taught formal/traditional grammar. It therefore seems to me that there is a link between formal grammar teaching and high levels of literacy. And that "prescriptive" rules based teaching is better for literacy than "descriptive" (or "progressive") grammar
The most recent PIRLS report about literacy levels for 9 and 10 year olds has the UK coming in 4th, with only HK, Russia and Singapore (I think) coming ahead of the UK. This is a pretty fantastic result, especially given that in the 2006 PIRLS report, the UK came in at 19th. The national curriculum introduced far more grammar into primary school teaching in 2016. It therefore looks like this has been significant, and reinforces the idea that prescriptive/formal/traditional grammar and language teaching leads to significantly higher levels of literacy?
Questions remain over how well language is being taught in senior schools and also what could be done about the generations which have less high levels of literacy. Many senior schools do not seem to be following the new, more rules based, national curriculum successfully.
There are also academic disputes about the use of phonics, about the fact that where phonics is used exclusively over presciptive teaching of grammar or to too great an extent there appear to be lower levels of literacy, eg in the states, according to some articles I have seen. Phonics as a basic start to reading is helpful, but there is so much irregularity in English, having phonics as the only teaching method is going to cause problems.
This is just my thinking on the subject, after having read the answers to my original question and then looked into it a bit more. If anyone has more knowledge or expertise over the link between formal grammar and literacy I would be really interested to know.