Tnank you, Bide. Could you just clarify some points for me, please?
1) Here you say,
As your original question was about commas, I would recommend you read the work of R L Trask, late Professor of Linguistics of the University of Sussex. He remains unsurpassed in clarity, conciseness and precision. His simple 'Penguin Guide to Punctuation' will, I'm sure, answer all of your questions on the subject..
My clarity, conciseness and precision are a bit dodgy!! (Especially at this time of night)
I'll get back to you on the other stuff.
"There are two relative clauses in these sentences, according to your words, but still, in the first case you recommend using commas and in the second one you don't. What's the difference?"
The difference is a difference in meaning which stems from the interpretation of 'which are situated there' as a restrictive relative clause. To quote Prof. Trask: "A restrictive relative clause is required to identify what is being talked about, and never receives bracketing commas. A non-restrictive relative clause is not required for identification, but only adds further information, and it always receives bracketing commas."
You can quite correctly argue, 'The buildings belong to the postwar period.' is a good sentence, and so 'situated there' is not a restrictive relative clause. However, you are now talking about all (the) buildings, not a specific set of buildings identified as 'situated there'. The sentence now has a different subject.
There used to be a convent, (which was) built in 1813, on this corner. There probably weren't too many convents built 'on this corner' 'in 1813', 'built in 1813' is not a unique identifier, not essential, so you should use bracketing commas around the non-restrictive relative clause.
I see that now. Appreciate your help, Bide. Actually, I was told about restrictive and non-restrictive relative clauses at the University, but have quite forgotten about it to this moment. The clarity came when you named them.
The name of the book you recommended seems pretty familiar... Would be interesting to read it. If I find the book, of course.