I am reposting this from the bottom of the debate on the progressive form of verbs because I would really like to get a variety of opinions on this. My take on grammar is as follows;
If you listen to interesting content in the target language intensively, at least one hour a day, then repeatedly read the same material; if you then systematically and consistently save and learn the key words and phrases you will need; if you try to write using these new words and phrases and have your writing corrected, and have incorrect phrasing replaced by better phrases;if you read a lot; if you do all of this you will learn faster than by trying to master abstract rules of grammar.
Grammar can be an occasional reference, but mostly it is a distraction that appeals to the "intellectual" mind of certain teachers and learners but is simply discouraging to most language learners. It is a theoretical construct. Time spent learning and debating fine points of grammar would be better spent listening and reading more and then talking to people in the target language.
I have wondered what role grammar plays in language learning. I think when learning a new language, you should not learn abstract grammatical rules but patterns. If you are a teacher, instead of saying that "I have caught three salmons today" is Present Perfect tense, you can give your students an authentic text and make them look for sentences with "have/has + third verb form" structures. Then you can proceed by making them aware from the context what actions are expressed by such sentences, and at a later stage you can contrast them with sentences in Past Simple, without mentioning any of these gramatical terms - what is there is just the examples. And at this stage they don't have to make up sentences of their own. However, you can occasionally ask questions that prompt the use of the "have + third form" structure (open-ended ones, of course). In this way they master "grammatical" rules unconsciously, in a similar way as native speakers learn to master them, though with slightly more guidance. On the other hand, I think that discussing issues of acceptability of certain structures does make sense at an advanced level, but the focus should not be on "grammatical correctness" but on whether a certain structure is actually used by native speakers or not.
You can learn any language without studying the grammar; after all, we all do this when learning our native languages. However, keep in mind that it's a slow process ... children take more than a decade to become truly fluent in their native languages, through trial and error.
Studying grammar greatly shortens the time required to construct sentences correctly and understand them when they are encountered. Similarly studying the phonology of a language greatly shortens the time required to speak intelligibly and understand the speech of others. Studying these things isn't a waste of time; on the contrary, it's a shortcut to language proficiency. With the right combination of practice, trial and error, and formal study, you can learn a language in the same family as yours (that is, another European language, if your native language is European) in about two years of full-time learning. Using just total immersion and trial and error alone, you can still learn a language, but it may take a decade or more to become truly fluent.
The notion that grammar is unimportant is seriously mistaken. The only real question is how best to acquire a knowledge of grammar. There is no doubt that such knowledge is essential to proficiency in any language.
I don't think I would ever have learned any language other than English if it weren't for the grammatical explanations that went along with it. It would have taken me ages to figure out that, for example, ia and aba both indicate the same tense and aspect in Spanish, but some verbs take one ending and other verbs take the other. Having someone already do the work of putting it all together in verb charts certainly saved me lots of time.
I do realize, however, that everyone has a different learning style, and that it is possible to learn a language without getting too deep into the grammar.
I think you should find a "golden mean" in dealing with grammar. Too much preoccupation with grammatical correctness can lead to anxiety and inhibitions, while on the other hand of course grammatical descriptions help organise your knowledge of the language in a very handy way. So I think when learning a language it is best to follow a "usage first, grammar second" method. This means that if you are a student, you should take a written text, for example, and detect as many examples of a given grammatical structure in it as you can, and look up their usage in a good book on grammar. On the other hand, if you are a teacher, you can focus on usage on your classes, and refer students to grammar books or overviews which they can study outside the class. I think you have to focus your attention on one or the other grammatical structure when learning a language, but the emphasis should not be on the rule but on examples of usage.
I think the more advanced level you are on, the more conscious attitude to grammar you can adopt. You should not bother too much with grammatical terms at elementary level, but rather focus on sentence patterns instead (as used in everyday speech). You can practise grammar in a more focussed way to a pre-intermediate student, after the main patterns have been thoroughly mastered. On more advanced levels you can discuss issues of which structure to use in a given context.
Finally, I think the age you start learning a language in is also important. Young children can do with very little grammar at elementary level, if they are given a lot of input and practice, but an adult will of course adopt a more conscious attitude, and will want to know more about how grammar works. But even in the latter case you should use as authentic examples as possible, in context.