Search for the same Japanese word twice and you may see it spelled two different ways. That is not a mistake. There are competing systems for writing Japanese in the Latin alphabet, and which one a translator picks changes the spelling.
The three systems
Three romanization systems are in regular use, and they disagree on the same sounds.
Hepburn
Built around how the words sound to an English speaker. This is what you see on station signs, passports, and most published translation. If you only learn one system, learn this one.
Kunrei-shiki
The system taught in Japanese elementary schools and used in some official documents. It maps more neatly onto the kana chart but looks strange to English readers.
Nihon-shiki
The oldest of the three and the most regular. It is rare outside linguistics, but you will run into it occasionally in older texts.
Where they split
The clearest differences show up in a handful of sounds. Same kana, different spelling:
Hepburn: shi / Kunrei: si
Hepburn: chi / Kunrei: ti
Hepburn: tsu / Kunrei: tu
Hepburn: fu / Kunrei: hu
Hepburn: ji / Kunrei: zi
So the surname written しみず comes out as Shimizu in Hepburn and Simizu in Kunrei. Both point to the same name. A translation tool will usually give you Hepburn, which is why that spelling dominates.
The long-vowel problem
Japanese has long vowels that double the length of a sound, and there is no single agreed way to show them. The city とうきょう can appear as Tōkyō with macrons, Toukyou spelled out, Tokyo with nothing, or Tohkyoh in older passports.
Osaka, Ōsaka, or Oosaka
Tokyo, Tōkyō, or Toukyou
Everyday English drops the marks: nobody writes Tōkyō on an envelope. For language study, the macrons matter, because a long vowel can be the only thing separating two words. おばさん (obasan, aunt) and おばあさん (obaasan, grandmother) differ by one held vowel.
One spelling that trips everyone
The topic particle は is pronounced "wa" but written with the kana for "ha". Hepburn romanizes by sound, so it becomes wa. The same goes for へ as a direction particle, written as the kana for "he" but romanized "e". If a tool gives you "ha" where you expected "wa", it romanized the spelling instead of the pronunciation.
Which one to use
Write Hepburn. It matches signs, official documents, and what readers expect. Keep the macrons when you are studying or when the long vowel carries meaning, and drop them in casual writing where Tokyo and Kyoto read just fine.
Try a sentence in the translator and compare it with the examples above.
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