The word "女孩子" is formed by attaching the character 女 (female) to the existing word 孩子 (child), where 子 acts as a nominal suffix within 孩子; this compound specifies "female child", yielding the meaning "girl".
Example Sentences
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那个女孩子很可爱。
Nàge nǚháizi hěn kě'ài.
That girl is very cute.
女孩子喜欢唱歌。
Nǚháizi xǐhuān chànggē.
Girls like singing.
这个女孩子在看书。
Zhège nǚháizi zài kànshū.
This girl is reading a book.
那个穿红衣服的女孩子是我妹妹。
Nàge chuān hóng yīfú de nǚháizi shì wǒ mèimei.
That girl wearing red clothes is my younger sister.