1. The present continuous is mainly used for fixed arrangements. For example: I'm meeting my friends at 2 p.m. She's having her operation tomorrow. We're flying to Paris next week. In most cases, there are other people involved in these arrangements - the friends, the hospital, the airline etc. The meeting with friends is definite, and the operation and the flight have been booked. 2. The 'going to' future tends to be used more for things which aren't arranged, for example: It's going to rain this afternoon. Listen carefully. I'm going to tell you something important. Don't eat all that chocolate. You're going to make yourself ill. As you can see, none of these things are arrangements, so you have to use the 'going to' form rather than the present continuous. Here's a tip - if in doubt, use the 'going to future'. In the first set of examples, it's possible to use 'going to' : I'm going to meet my friends at 2 p.m. She's going to have her operation tomorrow. We're going to fly to Paris next week. These sentences sound less definite, but they are still correct. However, the present continuous future cannot be used in cases where the 'going to' future is required. For that reason, the 'going to' future is a much safer choice. I hope that helps.

Going to for future is when you have made a plan or have an intention to do something. Present continuous for future is similar but the plan is an arrangement with someone else - they know about the arrangement too. I'm going to see John in hospital - he doesn't know I'm coming but this is my intention/plan. I'm seeing John this weekend - he knows that I am coming ; we arranged the meeting together. If we think of 2 sentences that are the same but change the verbs, you can see that one doesn't make sense. E.g. I'm going to buy some new shoes this weekend. . The second one is a bit strange because you haven't arranged to do this with someone. If we added 'with Sally', (I'm buying some new shoes this weekend with Sally), it makes more sense because (presumably) Sally knows about this arrangement for the weekend.