Reddit reviews Schaum's Outline of Spanish Grammar, 6th Edition (Schaum's Outlines)

I have done the entire Spanish tree on Duolingo, so I can confidently say that, yes, Duolingo will drill you in all the basics of Spanish. You can organize your Spanish study program around it. However, you will need to supplement it with other materials because although Duo drills you on grammar, it doesn't teach you grammar. Something to drill you on grammar is priceless, which is why Duolingo is so important, but it's not sufficient by itself.

I have written a long blog post about what I think are the best ways to supplement Duolingo but a short summary is that you need to buy a reference grammar of some kind, such as like Schaum's Outline of Spanish Grammar and you should really, really make heavy use of flashcards. I use Anki (which is free) and I have a technique that I think optimizes Anki for Duolingo.

Best of luck!

For grammar, I found Schaum's Spanish Grammar good for explanations exercises, and Conjuguemos good for practicing conjugations. Check your local library for Schaum's to save you a few books

The Schaum is a pretty good grammar text book with exercises throughout.

A linguistics text on Spanish won't really be good for learning the language, but as you encounter aspects of the language you find interesting you can find relevant linguistics books/articles which you can research further. E.g.

• Distinction between types of past subjunctive, the decline of the future subjunctive
  
• The Disappearing Spanish Verb Form in -Re, Leavitt O. Wright
  
• http://www.wikilengua.org/index.php/Pret%C3%A9rito_imperfecto_de_subjuntivo
  
• Terminal letters as indicative of noun gender
  
• A Simplified Approach for Teaching the Gender of Spanish Nouns, John J. Bergen
  
• A Frequency-based Analysis of the Norms for Spanish Noun Gender, Jens H. Clegg
  
• Phonology (and orthography) changes over time and between dialects
  
• Seseo, ceceo, distinción, heheo, yeísmo etc
  
• From Latin to Spanish: Historical phonology and morphology of the Spanish Language, Paul M. Lloyd
  
• https://en.wikipedia.org/wiki/Spanish_orthography#Older_conventions
  
• https://en.wikipedia.org/wiki/Spanish_phonology#Dialectal_variation
  
• Clitic agreement
  
• On the Absence of Spanish Past Participial Object Clitic Agreement: the AGRo Parameter in Romance, Jon Franco
  
• Spanish Clitics and Participles: A Historical Perspective, A. K. Barry
  
• Contrast in modern usage of cognate verbs with other romance languages
  
• Explanations for Language Universals, Brian Butterworth, Bernard Comrie, Östen Dahl
  
• History of 2nd person pronouns, conjugation, and modern dialectical variations
  
  
  etc.
  
  Wikipedia and Wikilengua are quite good starting points too.

Schaum's Outline of Spanish Grammar is an old-school, inexpensive, workbook which may help. Plenty of structure, solid explanations, tons of practice.