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 grammatical and syntactical terminology?
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.
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 ‘preterite’, 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:
The Simple English Wikipedia article on Syntax, and
I learned to speak conversational Spanish in six months using TV shows, movies, and even comics: I then wrote a book on how you can, too
I have a whole method and a book I wrote about it called The Telenovela Method where I teach you how to learn Spanish from popular media like TV shows, movies, music, books, etc. that you can all find online for free. It was the #1 new release in the Spanish Language Instruction section on Amazon for nearly a month after it came out and currently has 17 reviews there with a 4.9/5 stars average. It's available for $7.99-$9.99 for the e-book version depending on who you buy it from (Kindle version on Amazon is now $7.99) and $16.99 for the paperback (occasionally a bit cheaper, again, depending on who you buy it from).
It's currently available in both e-book and paperback from: