Polite and casual: two ways of speaking

Japanese has two modes, and you choose the mode by who you are talking to. The polite mode ends sentences in ます and です: たべます, がくせいです. The casual mode uses the verb's plain form and drops です: たべる, がくせいだ or just がくせい. Same words, same order — only the ending differs.

Who gets which? The teacher, shop staff, adults you do not know, anyone you just met: polite. Friends, siblings, people your age you know: casual. Unsure: polite. It is never rude, only a little formal, and it is the mode the whole course uses until the lesson on talking with friends.

Names follow the same line. さん after other people's names is polite and suits anyone; the teacher gets せんせい instead. Close friends and children you often hear with ちゃん or くん. Your own name never gets anything.

Two more things. The short form is not “wrong” Japanese: it is how Japanese people talk at home, and it is the form you find in the dictionary. And there are more modes than two — an even politer language for customers and bosses. That comes much later; ます and です carry you all the way through N5.