Question

1. Also many events and festivals are held with (the) opening of the sea.
(Is it correct? Do we ahve to sue 'the' here?)

2. The beach provides tourists with other attractions every year.
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Are they grammatical?

Answer 1

I don't understand what you mean by "with (the) opening of the sea" -- I guess because in my mind, the sea never closes.

2 is fine.

Answer 2

After the opening of the sea, people can swim....though people can use the beach all the year round. (So in this case, can we use 'the expression' above? Do we have to put 'the' before 'opening'?)

Answer 3

I see what you mean now.

Yes, 1 is fine as long as you use "the" before "opening." The word "the" indicates a particular event or action.