TheVinster wrote:

I just care that I said something, and that it's probably right.

Understanding grammar turns that probably into a definitely. You'd be surprised how many mistakes you notice from native speakers if you know even the basics of tenses and the parts of speech.

It's pointless trying to liken first and second language acquisition. Your brain is in a different stage of growth and your circumstances are completely different, no matter how much you try to make a good immersion environment.

You can learn grammar. You can learn to make perfectly constructed sentences that will probably sound retarded to native speakers. If you know how to form a structure, then you can use your immersion to get a sense of when it is and isn't appropriate to use it, so that eventually you don't sound retarded.

Writing a paper is a learned skill in any language.  Using a language grammatically is more of an instinct. 

No you don't have to study a language's grammar to be able to use the language grammatically.  Obviously there are illiterate people and kids who haven't been exposed to grammar classes who can use their languages just as well as those who have studied language extensively.  However, to get to the point where you can internalize a grammar, you need to be able to understand most of what you hear.  Linguists call it comprehensible input, but it's a pretty intuitive concept.  If you've ever read a word you hadn't seen before, but sort of understood what it must mean from context, it's the same idea.

Young children learn from comprehensible input with a lot of help from their parents and (literally) several years of passive, yet constant, observation.  Most of us grown folks, we simply don't have the time or the willpower to re-create that sort of an environment.  That's why a lot of people generally study some grammar at first, to get as close to the point of understanding as we can so that when we do start on the immersion phase, we can make solid progress. 

Just my thoughts.  There's many different ways to go about it, of course.  My view is just that adults have trouble learning the basics through observation and passive input because when it comes time to speak, they don't have any confidence in how to phrase it and have too much fear of an embarrassing mistake.

I don't think there will be a moment when it clicks for you. Maybe you might have an epiphany moment where you just realize you know whats up.

This usually happens after years and years of exposure, reading, and conversation.

I don't think it ever really just "clicks". I've only been studying Japanese for the past two years or so, but I've been exposed to it since I was a kid, and despite that there are still times where I struggle with grammar terms I thought I knew very well. As wildweathel stated, it's a gradual progression; you start out understanding nothing, and eventually, overtime, you build up to a native level.

If grammar was something that just naturally "clicked" at some point, then any native English speaker could pick up a 18th century novel and start reading away, with little to no difficulty. Despite any complicated grammar usage or complex verbiage, if grammar just "clicked", then all speakers would be able to understand all usages, because they would come to understand the patterns, linguistically. But it just doesn't work that way (for most people).

We come to understand our native language's grammar because we become familiar with patterns that we are constantly exposed to. But when you're learning a second language, unless you want to take 10+ years to reach a native level, just like with vocabulary, you can't learn grammar in the same way you did as a child. You need to use shortcuts, and learning grammar rules is one of those shortcuts. So while you can pick up grammar through constant exposure, it will, in my opinion, take a really long time to sink in before you can understand it and use it naturally. By learning grammar rules, you can give yourself a step up in comprehension.

That being said, I don't think you need to know what a pleonastic substantiative is or anything, but I do think it's important to at least understand the general concepts, even if you don't know what the name of the grammar term is. You don't need to know that 寒い（さむい） or 青い（あおい） are adjective, but you do need to know that they describe the qualities of an object or place; you don't need to know that を indicates the direct object, but you do need to know that it the word before it is receiving the action of the word after it.

I don't think it really clicks, does it?

I remember getting Cs and Ds in English when most my time on the internet was spent on MSN and Runescape (I think my English was at a disadvantage since I was living in a non-English speaking country). But when most of the internet time went to reading newspapers and forums where at least 80% of posts follow common grammar rules, my grades went up to A's and A*, I would be disappointed if I got a B.

I guess it may be different since I was fluent in English to begin with - but if you were to get to an advance level in Japanese wouldn't the grammar/spelling flow naturally in the same way after lots of reading?