` 1. ` It’s very important to keep the environment clean.

` + ` V-ing đứng đầu ` -> ` câu chia số ít

` @ ` Keeping the evironmet is very important: Giữ môi trường sạch rất quan trọng ` -> ` Viết lại với ctrc: It's adj to V

` -> ` It's adj to V: nó ... để làm gì

` -> ` keep sth clean: giữ thứ gì sạch

` -> ` the + N (xác định)

` 2. ` I haven't spoken to her for three years.

` -> ` It's + time + since + last + V`2`/V-ed

` => ` S + have/has + (not) + V`3`/V-ed + for time