I read an article on the National Geographic website:
In this sentence:
We reach the water and I try to swim – but the water is so salty I just float on the surface.
Cannot we use "that" between "salty" and "I"? If so, why has it been omitted?
In declarative content clauses, "that" is sometimes obligatory, sometimes optional, and sometimes inadmissible. It's obligatory where the clause is subject of the matrix clause, as in "That I need help is clear" or when the clause is preposed so as to precede the subject, as in "That I need help I can't deny". It's inadmissible in a clause that is complement to a preposition, e.g., *"I left before that he arrived". Elsewhere, it's generally optional, though it is more likely to be omitted in informal than in formal style, or after short and common verbs than longer and less frequent ones.