Paying over the Internet isn't always safe.这句话对吗?如果对,pay为什么变

Paying over the Internet isn't always safe.这句话Pay为什么变成ing形式? 求英语高人指导!Paying for the Internet isn't always safe.为什么是paying It isn't always safe to pay over the Internet同义句 1.I pay for the T-shirt over the Internet.对over the Internet Shopping on the Internet isn't always safe(改为同义句) Shopping on the Internet isnˊt aiways safe 改为同义句 （）（）（）（）sho .out of the shop without paying.这句话中paying是什么用法,为什么要加ing There isn't a bank between the hotel or the hospital这句话对吗 the car over there isn't 为什么用mine The box isn't empty.Empty it.这句话对吗? what a fine day,isn’t it?这句话对吗 英语翻译：The Grass Isn't Always Greener