The boy’s parents were very __ ___(thank) to the policeman because of his help.答案：thankful，整句话的意思为这个男孩的父母因为警察的帮助非常感谢他。也可翻译成“这个男孩的父母对于警察的帮助很感激”。
thankful前面有be动词were，用形容词形式
thankful
