Polite Speech Style (desu/masu) — I can use a polite speech style (desu/masu) consistently in common conversations for my level.

Historical Context & Motivation

Japanese is one of the few languages in the world where your choice of verb ending reveals your relationship to the person you're speaking with. The polite speech style, known as desu/masu-tai (です/ます体), didn't appear overnight. It grew out of centuries of social hierarchy, where how you spoke to someone could mean the difference between gaining a patron and causing serious offense. Understanding this history helps explain why polite speech is still the default style for any Japanese learner today.

So why does this matter to you as a high school student learning Japanese? Because desu/masu is your social passport. Speaking in plain (casual) form to a teacher or stranger would sound rude or overly familiar—like calling your principal by their first name on day one. The question this lesson addresses is: how do you consistently apply desu/masu endings so that your Japanese sounds natural, respectful, and appropriate in any common conversation?

Core Principles of Polite Speech

Before diving into conjugations, it helps to understand the foundational ideas behind polite speech in Japanese. These principles will guide every sentence you build.

Visual Explanation: Plain vs. Polite

Notice the pattern in the diagram: every sentence type has a clear conversion rule. For verbs, you replace the dictionary ending with the -masu form. For nouns and na-adjectives, you swap だ (da) for です (desu). For i-adjectives, you simply add です (desu) after the adjective without removing anything. This visual should become your mental checklist every time you form a polite sentence.

How Masu-Form Conjugation Works

Verbs are the heart of Japanese sentences, and converting them to the masu form is the most important skill for polite speech. Japanese verbs fall into three groups, and each group follows a slightly different rule for creating the masu form. Let's break down the mechanics.

Group 1: Ichidan (Ru-Verbs)

Ichidan verbs (also called ru-verbs) are the easiest to conjugate. Their dictionary form ends in -る (-ru), and to make the polite form, you simply drop the -る and add -ます. For example, 食べる (taberu, "to eat") becomes 食べます (tabemasu). Another example: 見る (miru, "to see") becomes 見ます (mimasu). The stem never changes—just swap the ending.

Group 2: Godan (U-Verbs)

Godan verbs (also called u-verbs) require one extra step. Their dictionary form ends in a sound from the "u" column of the hiragana chart (e.g., -く, -す, -む, -る). To make the masu form, you shift the final syllable from the "u" row to the "i" row and then add -ます. For instance, 行く (iku, "to go"): く (ku) shifts to き (ki), giving you 行きます (ikimasu). Similarly, 話す (hanasu, "to speak"): す (su) shifts to し (shi), giving you 話します (hanashimasu).

Group 3: Irregular Verbs

There are only two truly irregular verbs in Japanese: する (suru, "to do") becomes します (shimasu), and 来る (kuru, "to come") becomes 来ます (kimasu). These must be memorized since they don't follow the patterns of the other groups. The good news is that there are only two of them, and they appear so frequently that you'll remember them quickly.

Negative, Past, and Past-Negative Masu Forms

Once you have the masu stem (the part before -ます), you can create all four tenses of polite speech. The positive present is -ます (-masu). The negative present is -ません (-masen). The positive past is -ました (-mashita). The negative past is -ませんでした (-masen deshita). For example, 食べます → 食べません → 食べました → 食べませんでした. The same pattern works for every verb once you know its masu stem.

Desu with Nouns, Na-Adjectives & I-Adjectives

While masu handles verbs, です (desu) is the polite copula that handles everything else—nouns, na-adjectives, and i-adjectives. Understanding desu's four tense forms and how they interact with each sentence type is essential for consistent polite speech.

A critical point to notice is the difference between how i-adjectives and na-adjectives work with desu. For na-adjectives and nouns, desu acts as the main conjugating element—it changes to でした, じゃありません, and じゃありませんでした. But for i-adjectives, the adjective itself conjugates (おいしい → おいしくない → おいしかった → おいしくなかった), and then you add です at the end simply for politeness. This means you should never say おいしいでした (oishii deshita) for the past tense—the correct polite past is おいしかったです (oishikatta desu).

Worked Example: Building a Polite Conversation

Let's walk through a realistic scenario: you're introducing yourself to a Japanese exchange student's host family for the first time. We'll convert each casual thought into polite desu/masu speech step by step.

Notice how every single sentence ends with either です or します. That's consistency. If you had said 「日本の料理はおいしい」 without です in the middle of this conversation, the host family would notice the sudden shift to casual speech, and it might feel odd or even impolite.

When to Use Polite vs. Casual Speech

Knowing how to form desu/masu is only half the battle. You also need to know when to use it—and when casual speech is actually more appropriate. Getting this wrong can make you sound either rude or strangely distant.

Connection to Keigo and Advanced Politeness

Once you've mastered desu/masu, you'll eventually encounter keigo (敬語), the advanced system of honorific and humble speech. Keigo builds on the foundation of polite speech but adds two more layers: sonkeigo (尊敬語, honorific language) to elevate the actions of others, and kenjōgo (謙譲語, humble language) to lower your own actions. These are topics for more advanced study, but understanding desu/masu is the essential first step.

Don't be intimidated by keigo—it's a bridge you'll cross later. For now, consistently using desu/masu in everyday conversation is exactly what's expected of a Japanese learner at your level. Native speakers will appreciate your effort and understand that you're being respectful, even without keigo. Think of desu/masu as your solid foundation; keigo is the second floor you'll build when you're ready.

Practice Problems

Lesson Summary