already, but you might try doing some searches arunod the term 文体（ぶんたい）as that's the term for it.There are some Japanese linguistics books that talk about 文体 and when/how to mix them (and polite/regular language too, in speaking). I can see what I have at home.Some quick googling arunod "話し言葉　書き言葉　一覧" or 話し言葉　書き言葉　変換・例・ 文体 type things bring up various academic pdfs also, mostly aimed at people teaching Japanese, I've not looked in detail but you might find something there.More generally, now that you know it's an issue, you probably know that Japanese, like English, has native words which tend to be used in softer or more informal contexts and then adopted words (Chinese loanwords, or words later made up in that style) that are more "stiff" and used in more formal (or written) contexts. It's like the Anglo-Saxon vs. Norman thing in English. So you can just be on the lookout for it, and you'll naturally learn. (Same with the 文体 - read a variety of stuff, and just pay extra attention to what they use where. For spoken bits, read some fiction that will have direct quotes in it.)As just a general philosophy, it can help when you look up new words, or are just thinking about words, if you can come up with "the other style" equivalent for various words you learn. If you use a Japanese to Japanese dictionary, often the definition of a "stiff" word will have the "less stiff" version right there in the definition. Also, very often it's just using one or more of the kanji from the "stiff" word but with　訓読み. 譲渡 vs. 譲り渡す・譲る, that sort of thing. 渡米 vs. 米国に渡る、etc. FWIW in junior high we had a lot of practice at dissecting words like that.So practice reading newspaper headlines, perhaps, and thinking "how can I say this in simpler language? How would I say this to a 3rd grader?"