This is the 64th most frequent Thai word.
เข
"He" or "they" (third-person pronoun); can also mean "hill" or "mountain" depending on context.
In this sentence, 'เขา' is used as a third-person singular pronoun equivalent to 'he'.
เขาไปตลาดเมื่อวานนี้
He went to the market yesterday.
Here, 'เขา' is a possessive pronoun equivalent to 'his'.
หนังสือของเขาวางบนโต๊ะ
His book is on the table.
Again 'เขา' is a third-person singular pronoun, used in an interrogative context.