Question

Make sentences in the superlative form.
1: that is/BIG/book of the library
2: that radio is the/CHEAP/of the store

Answer 1

That is the biggest book of the library.
That radio is the cheapest of the store.

Answer 2

Olá!!
Para utilizar as formas do superlativo no inglês basta adicionar o -est no final. Há palavras maiores como, modern, que nao se pode colocar o -est no final. Então você usa "the most modern.."

1: that is the biggest book of the library.
2: that radio is the cheapest of the store.