A big obstacle for Japanese language learners must be understanding how to differentiate between 'tameguchi' (or 'tamego' in textbooks) and 'keigo'. Many of Japanese language learners may realize this concept when they get corrections, but there are situations where sentences are grammatically correct but unnatural due to the inconsistency of either 'tameguchi' or 'keigo'. While it’s often said that 'tameguchi' is casual and 'keigo' is formal, they don't seem to be exactly the same (considering 'keigo' is a unique concept in Japanese). There are 'keigo' expressions that sound casual. For instance, when it's inconvenient, one might say その日は予定が空いてないかもです to someone with whom they have a good rapport. Though it contains 'desu', 'teineigo', it sounds moderately casual. In my case, it’s hard to sense how my English expressions sound to native speakers. I believe this also applies to Japanese language learners.
Now, let me talk about something more intricate. I mentioned that there are situations where sentences are grammatically correct but unnatural due to the inconsistency of either 'tameguchi' or 'keigo', but this is mainly about written language. When it comes to spoken language, the situation is a bit different. Especially when Japanese people have verbal conversations, some blend 'keigo' with 'tameguchi' and vice versa. Are these expressions considered unnatural? It depends on the context, but they can be desirable. That’s because those who only use 'keigo' can come off as stiff outside of formal settings, while those who only uses 'tameguchi' can come off as shallow even in casual relationships. People who appear friendly and smart usually have a good command of blending them. This is maybe because such variation creates positive gaps, allowing others to perceive them in a multifaceted manner.
Frankly, I find it challenging for Japanese language learners to imitate such nuanced usages. Japanese native speakers normally switch between 'tameguchi' and 'keigo' in a way they change their tune on the condition that they know how their Japanese sounds to others. However, it's difficult for Japanese language learners to sense how their Japanese is taken. They're likely to "have accidents" when they do the same thing. On the contrary, if they blend 'tameguchi' and 'keigo' in a natural way during conversations, they must be a Japanese language expert.