We use 'I will' when we decide todo something at the time of speaking What would you like to drink? / I'll have an orange juice, please. Did you phone Ruth? / Oh no, I forgot. I'll phone her now. You cannot use the present simple ( I do / I go etc ) in these sentense. I'll go and shut the door. ( not 'I go and shut' ) We often use I think I'll ... and I dont' think I'll ... I feed a bit hungry. ..