你好
It often snows in winter in our city(改为否定句）-----------It doesn't often snow in winter in our city
It is sunny today in Nan Chang (改为同义句）-------------------it is a fine day today in Nan Chang. 或者 The sun shines brightly.today in Nan Chang
She is writing a letter to her pen pal （改为否定句）-------------------- she isn't writing a letter to her pen pal .
They are enjoying themselves（改为同义句）---------------They are having a good time .或 They are having fun .
供参考,希望帮到你