When 'have' means own or possess, you can't use it in progressive verb tenses. I have a car. (NOT I'm having a car) When 'have' means eat or drink, you can use it in progressive tenses. We're having coffee with our breakfast. What are you having for dinner? 'Have' is also used in many, many collocations (set phrases) and most of these can be used in progressive tenses. For example. I'm having fun. We're having a party tomorrow night. She's having a baby in the spring. They aren't having a good time. He's having a heart attack.

Most are okay, "I'm having a car" is a bit awkward. I can't think of a context in which such a statement really makes sense. There are a couple things to look out for. Firstly, this should be primarily used in situations in which 'have' is actually an action verb, rather than possession. Having a baby is an action, which could be otherwise described as giving birth or bearing a child. Having water with your meal refers to drinking water, rather than simply possessing water. Having an exam (most likely) means that you will host or give an exam. Additionally, "I'm having a car" sounds awkward because to 'have' a car is not an action. You also would not normally include the word 'am' in a statement of possession. I can't think of an action orientated meaning of 'have' which would make sense of car, and thus it sounds awkward. It should be noted that the reason I assume that you are the one giving the exam, is that that is an action, where as stating that you 'have an exam' is more of a passive state, as one is describing a schedule. ("I have a test tomorrow," "I have an appointment at 2," "I have no plan on Saturday.") The other thing to note is the placement of a 'to be' verb prior to have. "We -are- having water~" "I'm (I -am-) having a baby." etc. Note, that this form can be used to describe any for action verb to describe an ongoing action or plans for a future action (if context explicitly or implicitly notes it). I've focused primarily on future plans, as that is what your examples used. "I'm walking to school tomorrow." "We're eating dinner at 6:30." "He's starting his new job on Thursday." Try to pick out which of the following are correct or incorrect below: "We're having a discussion on ethics tomorrow afternoon." "She's having a birthday party for her cousin next Sunday." "We're having a new TV as soon as I get paid." "He's having his mother over for dinner." "I'm having an idea." "I'm having time on Monday." "We're having a trip to London in February."