How to Use ‘Desu’ and ‘Masu’ (And When to Drop Them)

If you’ve spent any time learning Japanese, the words desu (です) and masu (ます) are probably etched into your brain. They are the trusty sidekicks of every beginner, the polite little bows at the end of your sentences. You learn to say 「私は学生です」(Watashi wa gakusei desu – I am a student) and 「日本語を勉強します」(Nihongo o benkyō shimasu – I study Japanese). For a while, it feels like you can’t say anything without them.

But then you start listening to anime, watching Japanese TV shows, or talking to actual friends. Suddenly, you hear people speaking without these endings. Verbs are shorter, sentences feel faster, and you’re left wondering: did I learn it all wrong? When is it okay to drop the desu and masu?

Welcome to one of the most important transitions in your Japanese journey: moving from strictly polite speech to understanding and using casual speech. It’s not just a grammar change; it’s a social one. Let’s break it down.

The Foundation: What are ‘Desu’ and ‘Masu’?

First, let’s be clear about what these endings actually do. Desu and masu are markers of politeness. They belong to a category of speech called 丁寧語 (teineigo), which literally means “polite language.” This is your go-to, all-purpose polite form that’s safe to use in most everyday situations with people you don’t know very well.

The Role of です (desu)

Desu is often translated as “to be” (is, am, are), and it follows nouns and adjectives to make a sentence polite.

• これは本です。 (Kore wa hon desu.) – This is a book.
• 天気はいいです。 (Tenki wa ii desu.) – The weather is good.

Without desu, these sentences feel incomplete or abrupt in a polite context.

The Role of ます (masu)

Masu is an ending that attaches to verbs to make the action polite. You take the stem of the verb (often called the “i-stem”) and add -masu.

• 食べます (tabemasu) – to eat (polite)
• 行きます (ikimasu) – to go (polite)
• 見ます (mimasu) – to see/watch (polite)

This -masu form is what you conjugate for tense (e.g., mashita for past, masen for negative). It’s the backbone of polite verb usage.

Making the Switch: How to Drop the Politeness

The “casual” or “plain” form of Japanese is what you use with friends and family. In Japanese, this is often called ため口 (tameguchi). Switching to tameguchi involves changing how you end your sentences.

Dropping ‘Desu’ for the Plain Form

When you drop desu, you typically replace it with だ (da) after nouns and na-adjectives. After i-adjectives or with a sentence-ending particle like ね (ne) or よ (yo), you can often drop it entirely.

• 学生です (gakusei desu) → 学生だ (gakusei da) – I’m a student.
• 静かです (shizuka desu) → 静かだ (shizuka da) – It’s quiet.
• 大きいです (ōkii desu) → 大きい (ōkii) – It’s big.

Dropping ‘Masu’ for the Dictionary Form

This is even simpler: the polite -masu form of a verb reverts to its “dictionary form” (the one you look up in the dictionary).

• 食べます (tabemasu) → 食べる (taberu) – to eat
• 行きます (ikimasu) → 行く (iku) – to go
• 見ます (mimasu) → 見る (miru) – to see

Here’s a quick comparison:

Type	Polite Form (Teineigo)	Casual Form (Tameguchi)
Noun	犬です (inu desu)	犬だ (inu da)
Verb	飲みます (nomimasu)	飲む (nomu)
i-Adjective	寒いです (samui desu)	寒い (samui)

The Million-Yen Question: When Do You Switch?

This is the most critical part. Knowing how to switch is grammar; knowing when to switch is a social skill. The guiding principle here is the Japanese concept of 内 (uchi – inside) and 外 (soto – outside).

• Soto (外 – Outside): People you don’t know, superiors, customers, people of higher status or age. You are on the “outside” of their group. You use polite language.
• Uchi (内 – Inside): Your family, close friends, and sometimes close colleagues. You are on the “inside” of this group. You can use casual language.

When to ALWAYS Use ‘Desu’ and ‘Masu’ (Teineigo)

Rule of thumb: When in doubt, be polite. It is always safer to be overly polite than unintentionally rude. Stick with desu/masu when speaking to:

• Strangers: Clerks, waiters, people you ask for directions.
• Superiors: Your boss (上司, jōshi), your teacher (先生, sensei).
• Elders: Anyone significantly older than you (年上, toshiue).
• Acquaintances: People you know but aren’t close with, like a colleague from another department.
• Formal Settings: Business meetings, presentations, customer service interactions.

When It’s Okay to Use Casual Speech (Tameguchi)

Dropping the polite endings signifies closeness and intimacy. It builds rapport and shows that you feel comfortable with the person.

• Close Friends (友達, tomodachi): This is the primary context for tameguchi.
• Family (家族, kazoku): This is speaker-dependent, but most people speak casually with their immediate family.
• Your “In-Group”: Close classmates, teammates, or coworkers you have a friendly relationship with.
• People Younger Than You (年下, toshishita): An older person may naturally use casual speech with a younger person, but as a learner, it’s wise to wait for them to initiate it.

The Awkward Transition Phase

You don’t just decide one day, “I shall now speak casually to this person!” It’s a gradual, often unspoken, negotiation. Here’s how it usually happens:

1. Start Polite: You always begin a new relationship using teineigo (desu/masu).
2. Listen for Cues: Pay close attention to how the other person speaks to you. If a Japanese person who is your peer starts using the plain form with you, that’s a huge green light. They are inviting you into their “uchi” circle.
3. Reciprocate Gradually: You can start by mixing in some plain forms. Maybe use the dictionary form of a verb but keep the desu at the end of the sentence. Then, slowly phase out the polite endings as you both become more comfortable.
4. The Explicit Invitation: Sometimes, someone might just tell you! They might say something like 「ため口でいいよ」(Tameguchi de ii yo), which means, “It’s fine to use casual speech with me.”

Beyond ‘Desu’ and ‘Masu’: A Glimpse of the Politeness Iceberg

It’s important to know that teineigo is just one level of Japan’s complex politeness system, known as 敬語 (keigo). While you don’t need to master these right away, it helps to know they exist:

• 尊敬語 (Sonkeigo): “Respectful Language” used to elevate the other person (e.g., a customer, your CEO). It uses special verbs and prefixes.
• 謙譲語 (Kenjōgo): “Humble Language” used to lower yourself or your in-group, showing respect to the other person. This also involves a separate set of verbs.

The desu/masu form (teineigo) sits neatly in the middle. It is polite without being overly formal or humble, making it the most versatile and essential form for a learner to master.

Conclusion: It’s All About Connection

Mastering the use of desu and masu—and knowing when to drop them—is a major milestone. It signals that you’ve moved beyond memorizing phrases and are starting to understand the social dynamics of the Japanese language.

Don’t be afraid. Your Japanese friends and acquaintances will understand you’re a learner. Start with politeness, listen carefully, and don’t be afraid to take the leap into casual speech when the moment feels right. Getting it right isn’t just about sounding “correct”; it’s about building genuine, comfortable relationships. And that, after all, is why we learn languages in the first place.