In Chinese language, whenever a number (and/or this, these, that and those) is used with a noun, a measure word must be placed between the number and the noun. Unlike in English where we have countable nouns and uncountable nouns, the former needs just a number before the noun, while the latter uses a measure phrase after the number e.g. Four students and four loaves of bread.
Note: Measure words are also called Classifiers.
Gè is the most common measure and can be used with almost all nouns types.
Yī gè xuésheng – One student
Zhègè xuésheng – this student
Jiǔ gè yuè – nine months