"は" is written in the cases below.
1. particle "wa"
わたしはwatashiwa←わたし+は
※The sound of "は" changed other than initial. But the letter didn't.[pa]→[fa]→[wa]
※"へ" and "を" are the same.
"へ"[pe]→[fe]→[we]→[ye]→[e] (わたしは)へ[he]やへ[e] /にいく。I go to my room.
"を"[wo]→[o] (わたしは)お[o]んがくを[o]きく。I listen to a music.
2.word including particle "wa"
こんにちはHello←こんにちtoday+はis
これはこれはOh←これthis+は×2