Japanese orthography is mostly consistent when it comes to basic kana. A character in general will always be pronounced the same within words. There are, however, a few exceptions. The major one people find is the topic particle, written は, but pronounced the same as わ (/wa/). I'll also talk about its sister cases (particles へ and を).

Why?🔗

The reason is historical, relating to sound changes in the language. Until recent orthographic reform in the 20th century, kana orthography was actually quite a mess and had diverged from how the language was actually spoken. One example is what happened with the は行¹. Many words written with は now had it being pronounced わ, and many instances of ひふへほ had shifted into いうえお, respectively.

The reform standardized usage so that a few kana were obsoleted, and the sound↔writing relationship was better maintained. However, for 3 grammatical particles, their usage would remain the same. Those particles were は, へ, and を. In other words:

• は as a particle had come to be pronounced the same as わ.
• へ as a particle had come to be pronounced the same as え.
• を as a particle had come to be pronounced the same as お.

But the reform chose to leave them as is.²

How am I supposed to tell?🔗

Something that might worry beginners is: how is one supposed to know if は is actually /ha/ or /wa/ then? In reality, differentiating them is not really an issue as you get used to Japanese.

は is pronounced as /wa/ only as a particle³. As you get a better understanding of Japanese grammar it'll be easy to tell. At an early stage where you're reading stuff mostly written in kana it's a bit harder, but in a text with kanji it gets much, much easier.

Is it wrong to write こんにちわ?🔗

Now, I said it's pronounced this way "only" as a particle. This is both true and not. Some words have the は particle "built into" them due to their etymology, such that it's really not acting as a particle anymore, but you still write は though you say /wa/. This happens to be the case for こんにちは and こんばんは, which are very common words and a reason beginners might get frustrated by this は↔わ thing.

こんにちは, if translated very literally, only means "as for today". It's actually shortened from larger expressions, like "How are you today" or "It's quite hot today isn't it." So that は at the end is in fact the topic particle, it's just not actually acting like it because the expression has been shortened. So, it's "built-in," it's still written は but pronounced as わ.

Now, you'll actually see Japanese people writing こんにちわ online. That is, however, somewhat slangy. It's not considered the "proper" form, and this would only be done, say, as a sort of casual greeting in a chatroom or something.

So in a sense, writing it like that could be seen as a sign of fluency if you're doing it intentionally in the right context. Within learning communities, though, this should be avoided, because:

• Even if you are aware of what you're doing, a person of lower level might not be, and just end up adopting it without understanding.
• You'll often be corrected, even if you didn't need to be. Because there are many beginners that do this as a mistake, it's natural that this be pointed out and corrected. It's hard for those around you to know if you know what you're doing.

Is を ever pronounced /wo/?🔗

Basically, no. You may hear it like that in songs, but people don't do that when actually speaking. を is now only ever used for the particle. When representing a /wo/ in words imported from other languages, that's usually ウォ, not ヲ.

If you ever do see ヲ (or を) as part of a word for some reason (e.g. in ヲタク), it would still be pronounced /o/.

The obsolete kana🔗

This is more trivia than anything, it's not super relevant, but some learners find it interesting or end up being surprised when they first learn about this. As I said earlier, the orthographic reform obsoleted some kana. Those are:

• ゑ, in the table position for "we," and its katakana counterpart ヱ.
• ゐ, in the table position for "wi," and its katakana counterpart ヰ.

ゑ and ゐ, just like を, hadn't actually been pronounced /we/ and /wi/ for a long time. They had shifted to /e/ and /i/, so they ended up being redundant and made obsolete. The same thing would probably have happened with を if it hadn't been left around as a particle.

These kana no longer feature in any words, except when used stylistically for names. When you do see them, they'll always be read /e/ and /i/. The sounds /we/ and /wi/ in loan words would be written ウェ and ウィ.