英语句子,帮忙看看有没有语法错误.

第一句can已经是动词,后面的help不能再加 s.
第二句虽然很瘪嘴,但是勉强算没有语法错误,就是啰嗦又不顺.最好是说：
Only with enough knowledge can he help others with facility. This can also make the expert even more professional.
第三句,减to, met 不能是过去式要改成 meets, no matter 后面要加how
His zeal can help him get along well with everybody he meets, no matter how bad-tempered they are.
第四句如果expert指的是“专家”的意思,要说Politeness is essential to experts 或 Politeness is essential to an expert. 也就是要么前面加a为单数、要么后面加s为双数.如果你是想说“专业”的话,要改成：Politeness is essential to be professional.