คำลักษณนาม, also known as classifier or unit of measurement in English. Here are a few points to note:
1. All countable nouns have classifier.
2. Even though uncountable nouns do not have classifier, when placed in a container, it can then be countable.
This topic is not that easy. My suggestion is that do not memorise them, but to slowly learn. Every time you pick up a new noun, be sure to learn its classifier as well.
Originally the concept of classifier exists to describe characteristics of nouns. There are a few types:
1. Classifier to indicate type of nouns
Example:
Man/woman, human being, child, occupation – คน//khon

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Animal, furniture – ตัว//tua

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2. Classifier to indicate a group – this type actually exists in English. For example, we say a school of fish, a herd of cattle
Example:
A big group of animals – ฝูง//fŏong

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A group of people who forms a circle (playing takraw, musical instrument) – วง//wong

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3. Classifier to indicate how nouns look like
Example:
Round item (bangle, ring) – วง//wong

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Long and thin item (rope, thread, wire, even road) – เส้น//sên

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4. Classifier to indicate the quantity – this also exists in English.
Example:
Things that always come in pair (socks, shoes, fork & spoon) – คู่//khôo

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Dozen – โหล//lŏh

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All Metric/British unit of measurement – kilometre, kilogram, currency

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5. Classifier to indicate condition of nouns
Example:
Item that has been rolled (cigarette) – มวน//muan

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Items that have been tied up together (firewood) – มัด//mád

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