Be extremely cautious. Common sources include:
: Deep dives into tenses , the sequence of tenses, conditionals, and non-finite verbs like gerunds and participles. Pk De Sarkar English Grammar Book Pdf 80l
Focuses on traditional grammar rules that are essential for formal writing. Be extremely cautious
Many PDF files circulating online are scanned from physical books. Optical Character Recognition (OCR) software sometimes misreads page numbers, edition numbers, or file labels. "80l" could be a mis-scan of: the sequence of tenses