When to Pronounce Final h (and when not to)

You're in for a shock here I'm afraid - the rule is simple. But, at first glance, it can be confusing to recognize when a final [h], usually represented by th in the spelling, is pronounced and when it's not. The surprisingly simple rule is that when you have the final letters, like th, indicating a final [h], you pronounce the [h] if it's at the end of a monosyllabic word - that is, if the word has only one syllable and if the preceding vowel is short. Remember that a diphthong counts as a long vowel!

Note that even if the [h] is pronounced, it tends to be relatively weak and often disappears in normally fast speech, but more of that later.

Soo ... let's look at some contrasting examples:

cath	⇨	[kah]
bùth	⇨	[buː]
gath	⇨	[gah]
bàth	⇨	[baː]
tuath	⇨	[tuə]
mòrghath	⇨	[moːrɣa]

Having a longer word ending in -th is actually very rare, so the issue of counting syllables goes away for the most part anyway.

OK, you might ask whether confusion is created if bàth "drown!" and bà, the genitive of "cow", are pronounced the same way. Actually, yes and no. Yes in the sense that they're pronounced the same way, but no in the sense that it doesn't complicate things. For one thing, English has many more of this type of issue - think of [naɪt] and [naɪs]. You tell me whether those two are "night" and "gneiss" or "knight" and "nice"! And the other two reasons that confusion is rare are word order and context. In Gaelic you'll never get the word [baː] "cow" in the same position as you might get the word [baː] "drown!" because the first is a noun and the second a verb. Plus, there's always the ever helpful thing called context which usually clears up any confusion. All languages have ambiguity because we couldn't pun otherwise!

So what was that about fast speech? Well, this final [h] is just one of those things that tend to disappear when you speak normally. There's more on fast speech to be found here.