Well, yeah, the way I usually do it (pff, it's not like I learn a new language every day... but still, I think I'm starting to get some regularities), I have to learn the basics of grammar - how the noun looks, the verb, tenses, declinations, if applicable... although this time I actually don't know how most tenses work in Welsh and that kind of doesn't bother me. The way it seems to be working this time is that I come across an unfamiliar construction, start looking it up and end up discovering that it's how the past tense is made. I think my attitude is something like this: however weird and difficult the grammar may be, it won't match the monumental task of remembering at least 5000 words to be able to keep up with any text or conversation. And add to that the fact that native speakers sometimes get their own grammar wrong. It's probably happened to you, but I've kind of been paying extra attention how sometimes (or you could say, quite often) native English speakers forget even such basics as plurals. In the sense that at the end of a sentence they realize that they made a plural incorrectly (such as "sheeps") and then go back and correct themselves... I don't know, maybe that's not a valid reason. But that's kind of how I see it.

I suppose that's also particular to my focus. I haven't had to use grammar in a long time (lack of time, social anxiety... well, no obvious place where I could do it), and really, with Welsh, don't expect to. And I've been able to get away as an observer (or reader) in at least German with suspiciously little understanding of it.

I probably should immediately put a disclaimer that I in no way advocate or promote bad grammar and waive any liability in case you follow my advice.