The phrase "a terrible decline" is correct and usable in written English.
You can use it to describe a significant decrease in quality, quantity, or condition, often in a negative context.
Example: "The company has experienced a terrible decline in sales over the past year, leading to layoffs and budget cuts."
Alternatives: "a significant downturn" or "a drastic drop."
It sounds like a terrible decline in standards.
The advent of the Heath government signalled a terrible decline.
Since joining I have observed a terrible decline in the value of my job and the service which I used to provide.
Public funding of higher education has increased under New Labour, after a terrible decline under Margaret Thatcher, but it can't do the whole job for a greatly expanded university sector.
In one of his earlier short fictions, "Downfall of the Heart" (whose original title, Untergang eines Herzens, is a proleptic echo of the German title of Beware of Pity – Ungeduld des Herzens, or the heart's impatience), a self-made businessman succumbs to a terrible decline after seeing, or imagining he has seen, his daughter sneaking out of a man's hotel room in the middle of the night.
"[The UK] is a country in terrible decline," he said, tongue – we hope?
When will Team GB, Sport England (or whatever the gov agency is called now) wake up and put a stop to this terrible decline.
It has been painful to watch the terrible decline of Zimbabwe, a country so rich in human and natural resources.
Differences of opinion disappeared, and the APA suffered a terrible organizational decline.
Academics have warned that changes to the science curriculum are a "catastrophe"; the numbers pursuing other truly demanding subjects – especially modern languages – are in terrible decline; we've fallen down the OECD Pisa league tables of educational achievement; and, as we saw last week, primary school standards are stagnating.
Everybody knows the English public house is in terrible decline.
