Classifiers: counting words
Count people and things with classifiers.
A Thai noun isn't counted directly. A classifier, a measure word, goes with the number. กาแฟสองแก้ว is literally coffee two glass. English does this with some nouns, such as two cups of coffee or a loaf of bread. Thai does it with every noun you count.
two cups of coffee
three dogs
Two bottles of water, please.
The most useful classifiers
Some nouns are their own classifier: ห้องสองห้อง, two rooms. If you don't know the classifier, repeating the noun in this way will usually be understood, even where it isn't the standard choice. With one, the number often goes after the classifier: กาแฟแก้วหนึ่ง, a coffee. กาแฟหนึ่งแก้ว is also correct and puts more weight on the number: one cup.
Common mistake
Don't put the number straight before the noun. Two coffees isn't สองกาแฟ but กาแฟสองแก้ว: the thing, then the number, then the classifier. A missing classifier is one of the most noticeable beginner mistakes.
Practise
Kai says
Thai digits have their own shapes: ๐ through ๙. The number practice helps you connect those shapes with values you already know.