Primer3 Input (version 0.4.0)
http://frodo.wi.mit.edu/


Great for daily 30-minute habit-building, but pair it with a grammar workbook if you want full fluency.

If you find a free PDF, understand that you are missing 80% of the value. Buy the complete pack (book + CDs or digital audio) from Assimil or a major bookseller.

This resource is referenced here: