`4`.Nga is the tallest student in her class

`\rightarrow` So sánh nhất : S + is / am / are + the adj-est / the most adj

`5`.His sister is taller than him

`\rightarrow` So sánh hơn : S1 + is / am / ... + adj-er / more adj + than + S2

1,Nobody in Nga class is taller than her.

-> Nga is the tallest student in her class.

Dịch : Nga là học sinh cao nhất trong lớp của cô ấy

Form :

Short adj : S + tobe + the + adj + est + Noun/Pronoun

Long adj : S + tobe + the +adj + Noun/Pronoun.

2,He is shorter than his sister.

->His sister is more taller than he.

Dịch : Chị gái anh ấy thì cao hơn anh ta.

Form : 

Short adj : S + tobe + adj +er + than + Noun/Pronoun

Long adj : S + tobe + more + adj + than + Noun/Pronoun