Hello Cheuk Chan, In English words ( unlike Chinese ) have 'singular' and 'plural' forms. To form the 'plural' of a word referring to more than one nouns ,you usually add 'S' to the singular noun: computer computerS book bookS boy boyS girl girlS etc.... However if the word ends in s, x, z, ch, or sh, the plural is usually formed by adding 'es.' This is because when you add an 's' to the end of these words, you have to add an extra syllable to the the word in order to pronounce it as in : box boxES ( you can't say boxs , you won't be able to pronounce it easily). brush brushES buzz buzzES Apart from the above mentioned, irregular nouns, those that do not use the regular plural ending, do end sometimes in 'es' ,regardless from the above rule as for example : Tomato TomatoES not TomatoS Half HalVES ( here you change the 'f' into 'v' and add 'es') Calf CalVES Thesis ThesES ( here you change the 'is' into 'es') Basis BasES etc... You have to know about those irregular nouns, where you add 'es' when forming their plurals. The more you practice using them and reading them you will eventually be able to form them almost automatically.

对不起, on the sentences it wouldn't let me do what your teacher did, so I'm going to do it this way: he is singing. He is eating. He is rapping. Those are on the other examples. :)

:O I think your teacher forgot to mention why you add "ing" to a word, it puts a verb in present tense, for example "他在跑“ would not be "he is run", but would be "he is running". it means 他在做这个。不是他刚刚做这个， 也不是他就要做这个， 但他在做这个。 more examples: he is singing he is eating he is rapping