Indus was a large river in Pakistan.

Explanation

The sentence "The Indus is the largest river in Pakistan" is grammatically correct because it follows the subject-verb-object structure. "The Indus" is the subject, "is" is the linking verb, and "the largest river in Pakistan" is the predicate nominative phrase that describes the subject. It's also a factually accurate statement about the Indus River being the largest river within the geographical boundaries of Pakistan.