"Ha (は)" changes to "Wa" when it functions as a grammatical particle, rather than a portion of a word. The more vocabulary you learn, the more you'll be able to tell when it is meant to be used as a particle. As a particle it indicates the "topic" of a sentence, but not necessarily the "subject." Here are some example sentences using "Wa." はじめまして、私の名前はリーです。 今週は先週と比べて同じぐらい退屈しています。 僕のペットは車に轢かれて、なくなってしまいました。 Perhaps you won't be able to understand these sentences, but you might be able to tell where Ha become Wa in these sentences. If you are still confused, please let me know.

when used as a particle