The waiter teaches me a few words of English.
（若以现在时态的话,the waiter 是第三人称,teach 后面必须加 es,强调句可以加上 does）
The waiter taught me a few words of English.
（过去时态的表达）
The waiter did teach me a few words of English.
若加上 did ,是起到了强调作用.但后面就要变成原型动词,故 teach 去掉 es.