`1.` People mustn't post polite comments on social media.

`->` mustn't do sth: không được phép làm gì

`2.` It is becoming really popular to study online nowadays.

`->` It + tobe + adj + to V: thật...để làm gì

`3.` How long have you gotten on with your neighbor?

`->` How long + have/has + S + P2?

`1`. People musn't post polite comment on the social media.

`-` S + must not + V-inf: không được làm gì

`2`. It is really popular to study online nowadays.

`-` It's + adj + to-V

`3`. How long have you gotten on with your neighbor?

`->` How long + have/has + S + V-pII?