cooccurrence relations

[suprunp]

The second condition is most explicitly shown in
cooccurrence relations between the past tense and past time-position
adverbials such as last week, in 1932, several weeks ago, yesterday, etc:

Freda started school {last year. [1]
Freda started school {in 1950. [2]
(A Comprehensive Grammar of the English Language)

I understand 'cooccurrence relations' as 'relations that co-occur' (in one sentence?).
But why 'relations' - are there several relations between the past tense and past time-position adverbials?

Thanks.

[BobK]

You understanding is fine. Each example demonstrates 'the coocurrence relation'. 'Relation' is just a word that can frequently attract an s. Don't worry about it.

b