In some words, a written ห is silent.
You know ห as a real consonant with its own sound. But when ห is written immediately before certain other consonants, it is not pronounced at all. In หมา the word starts with the ม sound — the ห is not heard.
หมา
written, but not sounded · sounded
The silent ห lends the syllable its own class.
ห is a high-class consonant. ม is low-class. When the silent ห leads, the syllable follows the high-class rules instead of the low-class ones — even though the sound you hear is the ม.
Class is the first input to the tone rules, so changing the class changes the tone the spelling predicts.
One letter, a different word
These pairs sound the same except for the tone. The only thing written differently is the silent ห — and that is enough to change which word it is.
มา
Midม alone — low classto come
หมา
Risingwith ห — high classdog
นา
Midน alone — low classrice field
หนา
Risingwith ห — high classthick
ห leads 9 letters, and only these.
They are all sounds the voice can hold — the ones you met as live endings. None of them has a high-class letter of its own, so a leading ห is how Thai writes a high-class version of these sounds.
Reviewed words you will meet:
หมู
Risingpig, pork
หลาย
Risingseveral
หวาน
Risingsweet
หรือ
Risingor
หญิง
Risingwoman
หยุด
Lowto stop
A ห that is not followed by one of these letters is just ordinary ห with its own sound — as in ห้อง, “room”.
อ does the same thing, in exactly 4 words.
A silent อ before ย works the same way — but อ is a mid-class consonant, so these follow the mid-class rules, not the high-class ones. This is not a pattern you can extend: it is these four words and no others.
อยาก
Lowto want to
อยู่
Lowto be at, to stay
อย่า
Lowdon't
อย่าง
Lowkind, way
The one thing to keep
A silent leader lends the syllable its own class — ห is high, so ห-led syllables are high; อ is mid, so the four อย words are mid. When you work a tone out from the spelling, read the leader first: the class you feed into the rules is the leader’s, not the letter you can hear.