Question: When do we use these phrases? Answer: 'Will' and 'going to' both refer to future time, but they are rarely interchangeable. We typically use 'will' when we are making a decision at the moment of speaking. Often this is an agreement, an offer or a promise. For example: A: Can someone go and buy some milk? B: OK, I'll go. You use 'I'll' here because, at the moment of speaking, you are volunteering to do this. Then, a short time later, you might say this to another person: A: I'm going to buy some milk. Can I get you something? In this case, you say 'I'm going to buy' because decision has already been made. Perhaps you already have your coat on and your money in your pocket, and you are preparing to leave. That is a key difference between the two forms. As native speakers, we use these forms so instinctively that many of us are not even aware of how they are different.

I think you mean going to + verb in order to refer to a plan for the future that you have already made e.g. I have decided that I am going to visit Italy this year during my holidays. You can also use it for logical deductions from direct evidence e.g. There is a big dark cloud in the sky now. I think it's going to rain later. Will + verb is used for offers, promises, spontaneous decisions, and simple statements about future events. If you are not sure, write some sentences and we can check. There are also plenty of grammar websites which can explain the rules in more detail than me, and with exercises.

"going to" is slang for "will". Both are equally common.

"going to" and "will" aren't sentences. Look up the definition of a "sentence" and a "phrase". They are identical in meaning in most cases. There are some differences in a few cases. Look these up in the dictionary. This should give you some basic information.

They can be used interchangeably--for instance: "I'm going to wash my car." "I will wash my car." -- both expressing intent for future action. However, "going to" can also be used in another sense--for instance: "I'm going to the beach this afternoon." "That train is going to Chattanooga." -- expressing not intent but goal or direction.