There are two consonants which can be placed before a mid class consonant at the start of a word and can change the mid class consonant's class and so change the syllable's tone. They are อ and ห. When used like this they are not pronounced.
Considering อ first. It changes the class of the following consonant to mid. In fact, there are only four words which are like this, and all are pronounced with a low tone. They are:
|don't (plus verb)
|to be, remain
Many, many words have leading ห which changes the class of the following consonant to high. For example, แหวน (/wɛ̌ɛn/, ring), หมวก (/mùak/, hat) and หล่อ (/lɔ̀ɔ/, handsome).
Here are some words to practise reading:
Note that some words can be interpreted in more than one way.
For example, whilst แหว (meaning to scold) is pronounced (as one might expect) /wɛ̌ɛ/,
the very similar looking เหว (abyss) is pronounced /hěew/. หวง (to cling to) is /hǔaŋ/,
In some cases it can help to know that leading ห is only used before the sonorants, namely ย, ร, ล, ว, น, ม, ง (plus one other sonorant not yet covered in this course).