>Would japanese people consider me as a rude gaijin? Yes, I am sure they do. >I know all the textbooks say I HAVE TO use polite Japanese (for example masu instead of ru) when speaking to senpai or strangers, First, I don't agree with you and those textbooks that say "masu" form is a polite Japanese. I think it is not polite but USUAL Japanese. Look at all the answers and comments here written in Japanse by Japanse people. They all are written in "Desu/Masu" style, so-called "polite way". Then are we all so polite? I don't think so. We only use "desu/masu" because it is usual. Japanese speech style has 3 layers, "vulgar-usual-polite", like ”してもいい(か）？“, "してもいいですか？“, "さしつかえなければ～してもよろしいでしょうか？” for saying "can I do it ?". You don't have to use the polite one. But I am sure you have to use the usual one (because it is usual). >I feel much more confident and comfortable speaking casual japanese and it always takes me some time to think of the polite version how to say a certain thing. Gotcha, but still you have to use "desu/masu", again because it is the usual one.

ていねいな　ことばを　つかうべきだ　とおもいます。 ともだちに　なったら、カジュアルな　ことばを　つかって　いいです。 でも、せんぱい　や　としうえ　の　ひと　には、ずっと　ていねいな　ことばを　つかわなければいけません。 参考になれば幸いです。 Community tutor JJ.