In Chinese, when we talk about the number of people or things, we usually need a measure word between the number and the noun. In English, you can say "one book" or "three people" without anything in between — but in Chinese, the pattern is always number + measure word + noun. Tap the speaker icon next to each example to hear it.
個 is the most common measure word, and when you're not sure which one to use, 個 is often a safe choice in daily conversation. But some nouns pair with a specific measure word instead — we say 一本書, not 一個書; 一杯水, not 一個水. Don't worry if this feels difficult at first. Native speakers also learn measure words naturally, through daily use, not by memorizing rules. The best approach is to learn each new noun together with its measure word, as a single phrase — that's what will make your Chinese sound natural.