Frankly? None. You can learn all the grammar you’ll ever need for anything other than passing a test on grammar via context–meaning that you’ll learn how to say things correctly simply from trial and error: speaking with native speakers, making mistakes, and being corrected; you don’t need to know what it’s called when you correctly use the subjunctive, knowing what the subjunctive is is not a prerequisite to using it properly. When you speak English (or whatever your native language is), can you actually explain everything you say in formal grammar and syntactical terms? No, very few people can, but you and every other native speaker can still speak, read, and write at a very high level of fluency with very few if any grammatical errors, right? You’ll never need to know actual formal terms for the concepts that you’re learning, you needn’t know the difference between the preterit and perfect subjunctive…however, it may be worth the time it takes to learn a minimal amount of it simply so you can understand verb conjugation charts and any exercises you may do online or in workbooks if you’re trying to learn how to read and write the language–I would recommend that you know what the difference is between the present tense and the preterit tense, in other words.
A quick side note: if you’re interested in teaching yourself Spanish…
I have a short post and video (that are free to read and view of course, won’t cost you more than a few minutes of your time) on how to do precisely that with the system that I put together which allowed me to become fluent in Spanish in just 6 months after years of trial-and-error by watching Spanish-language TV shows (like telenovelas, hence the name of the system) and movies, reading Spanish books and comics, and listening to Spanish music. If this sounds interesting to you, check it out by clicking the link below (the following link should open in a new tab or window for you when you click it so I’m not asking you to leave this article here):
I also include some quick and valuable tips for learning Spanish as well as a couple of the most useful free Spanish-learning websites that I recommend.
However, if all you want to do is learn how to speak, then knowledge of this stuff is the least useful, and even if you’re interested in learning how to read and write it’s still only slightly more useful.
I’ll put it this way: the best way to determine what you need to learn is to simply dive into the language and then look up anything you find that you have to know as you go along. For example, if you’re using a workbook and they mention the ‘preterit’, and knowing what that word means is vital to you learning what you’re trying to learn, then go look it up, otherwise don’t. In other words, let necessity dictate whether you bother learning something or not.
If you want to just quickly learn the basics, two great resources that you can get through in under an hour and continually reference are:
A Quick Note Before We End…
I’ve got two posts that I’ve put up that I’m recommending everyone interested in learning Spanish go read if they haven’t already (if you have, ignore this, sorry): How to avoid wasting months learning Spanish the wrong way (basically this is my “how to get started right in learning Spanish” post for complete beginners) and The Telenovela Method where I cover how to use popular media like movies, music, and books to learn Spanish. Additionally you can check out the front page for a more complete list of my best and most popular posts.