This is the 1421st most frequent Chinese word.
a
In Chinese, "a" (啊) is a particle used to express affirmation, surprise, or emphasis.
In this sentence, '一' ('a') is used as an indefinite article indicating a single item—here, one 'teacher'.
他是一位老师。
He is a teacher.
Here, '一个' ('a') represents a single countable noun, specifically 'question' in this context.
我有一个问题。
I have a question.
In this example, '一个' ('a') is used to denote one specific instance of a fruit, which is an 'apple'.