I have a question about Japanese reports. I am studying at a university in Japan. And when I came here, I found out that Japanese people use ～だ instead ～です, ～たinstead ～ます and so on. And it seems rude to me. I'm afraid that I could misunderstand something and do it incorrectly. Are there any words I should avoid in the reports? How to use grammatical forms in reports? Which words to use and which not?
Yes, plain form is a standard for writing reports. In fact, 新完全マスター文法N3 has a chapter on this topic, on page 142. Here is what it says:
In order to ensure a unified tone, casual and formal language are not used together. You should also adhere to a single register, whether using 丁寧体 (です and ます) or 普通体 (だ or である) verb forms.
Formal language used in reports and articles should use the 普通体 (だ or である) forms. In addition, for continuity, the て form can also be replaced as follows:
In formal language, colloquially abbreviated forms (like みなきゃ) and omission of particles are not acceptable.
From there, the book gives a small table that compares the differences between expressions in formal language vs daily language:
"formal" vs "daily language"
- 非常に・大変 vs すごく
- 多く・大勢 vs いっぱい
- 少し vs ちょっと
- やはり vs やっぱり
- さまざまな・いろいろな vs いろんな
- 述べる・話す・言う vs しゃべる
- 行う vs やる
- しかし・だが vs でも・だけど
- なぜ vs なんで
- ～など vs なんか
- ～と・～という・～そうだ vs ～って
- ～ようだ・～らしい vs ～みたいだ
Still, at the end of the day, to get a detailed conclusive answer, you have to consider the standards of writing in your field of study. You can probably find books and style guides on that topic.