`1)`

`-` I am

`-` she is

`-` he is

`-` we are

`-` you are

`-` they are

`-` a cat is

`2)`

`(+)`

I am going

she is going

he is going

we are going

you are going

they are going

a cat is going

the children are playing

`(-)`

I am not going

She isn't going

he isn't going

we aren't going

you aren't going

they aren't going

a cat isn't going

the children aren't going

`(?)`

Am I playing?

Is she playing?

Is he playing?

Are we playing?

Are you playing?

Are they going?

Is it going?

Is a cat going?

Are the children playing?

`=>` cấu trúc cho thì HTTD

`(+)` S + tobe + V_ing + O

`(-)` S + tobe + not + V_ing + O

`(?)` (Wh) + tobe + S + V_ing + O + ?