How to say “Happy birthday” in Japanese

The polite form is お誕生日おめでとうございます (o-tanjoubi omedetou gozaimasu); the casual form is 誕生日おめでとう. The politeness does double duty here: the honorific お in front of 誕生日 and the ございます tail on おめでとう each add a layer, and the casual form drops both at once. To a friend, 誕生日おめでとう — or just おめでとう if the context is obvious — is the natural register; the full form belongs to teachers, bosses, in-laws, and anyone you'd address in ます-form.

The pattern generalizes well beyond birthdays. おめでとうございます is the all-purpose polite congratulations: attach it to 合格 for passing an exam, ご結婚 for a wedding, or say あけましておめでとうございます at New Year's. Learn the birthday version and you've effectively learned them all — the noun changes, the congratulations machinery doesn't.

Two production notes. First, the register mistake runs both ways: casual おめでとう to your boss reads as overly familiar, while the full formal version aimed at a close friend can sound like a greeting card instead of a person. Second, the phrase is long and vowel-heavy — おめでとう ends in a held long vowel, and clipping it short is the most common slip when learners finally say the phrase out loud instead of typing it into a birthday message.