Marcus, I learn languages by "osmosis" rather than by memorizing rules, I try to replicate the language learning of a child learning his mother tongue. As an adult I have the additional tool of knowing about grammatical rules, but I use that tool to explain to myself certain characteristics of the language which I nevertheless try to adopt as natural rather than as the result of conscious thinking. I have read hundreds of English language novels and other books, had countless conversations in speaking and writing and have probably seen or heard and therefore now can mimick just about every imaginable sentence structure there is. If things like a non-stressed "what did happen?" never turn up, then I never get an impetus to use that form. When I notice forms like "what did happen?" then I try to glean from the context what this form achieves in addition to a simple "what happened?". Usually there will be a dialogue, for example:
A: But that's not what happened!
B: Alright, so what did happen then, in your words?
And I simply understand from such examples sooner or later that the form with "did" adds emphasis.
If you approach a language only through rules and translation, then you never achieve real access to the language. It's like using only half your brain for the language. The left hemispere processes the foreign language, applying rules and translations, then transfers the syntactic content thus transformed into the native language to the right hemisphere which harvests the semantic content; and the semantic content of the reply from the right hemisphere is clothed in the native language first in the left, then again transformed by applying the set of known rules and translations, and hopefully comes out correctly. That's like an interpreted computer language: slow.
What you need to aim for is a direct connection between the syntactic processes in the left hemisphere to the semantic in the right hemisphere without a detour through your native language. Only then you will really be able to think in the target language, and then you will really know the language. This is more like a compiled language, and it is fast.
I'm still a couple of years at least from reaching that point in Russian, but I am quite content that I will, eventually.