Объясните , в чём разница например между: fred is buying a ticket и fred buys a ticket?

my mother is a doctor . she goes to work and treats children .

to be afraid ,to be hungry,to be busy,to be tired,to be cheerful,to quiet,to be kind,to be lucky

  to   go for a walk,to sit ,to stand,to read,to run ,to jump,to see , to drink,to write ,to think,