It can be confusing. The main difference is (as StevenSteven said), "the" is specific, and "a" is not specific. For example: "I'm going to a barbeque today." = You assume I don't know which barbeque. But if I knew which, because you'd told me about the barbeque earlier, then I think you're pretending that I don't know. "I'm going to the barbeque today." = You assume I definitely know which barbeque. You've already told me. But if this is the first time I hear about the barbeque, then I wonder why you assume I should know... and then wonder why I wasn't invited! Keep trying - even with mistakes I'll work out what you mean to say. Or I can always ask.

I think since English is the most widely spoken second language, native speakers are usually quite accustomed to overlooking common mistakes like these, in casual conversation. While there might be some cases where the meaning could be ambiguous, I think most of the time you will not be misunderstood if you mix your articles. In normal conversation, with non-native speakers, it's kind of expected that you will make some mistakes. So don't worry too much. If you're saying interesting things, people will be thinking about what you're saying and probably won't notice too much! :) When you're writing you should try to take a bit more care. This is just because it's somehow more harsh to read grammatical errors. They tend to interrupt the flow of reading, and unfortunately there are generally higher expectations for written correctness. I know there are a lot of exceptions, but in general just try to remember that "the" refers to some specific item, whereas "a" is referring to an unspecified item. So if you haven't mentioned it already, or it isn't somehow known to the listener then use "a" (or "an" if the next word starts with a vowel). If you're referring to the same thing later, or the listener knows about the item beforehand (either through common use or because there could only be one possibility), use "the". Since there are exceptions this won't be 100% correct, but it should be easy to remember and correct in the vast majority of cases. Examples: "Let's go to THE supermarket" - say, if it's a supermarket you always go to with the listener. "There is A party on tomorrow. THE party starts at 8" - first mentioning there is some party (A party) that is previously unknown to the listener. Then referring to this (now specified) party using THE later. "Don't look at THE sun" - since there is only one, it's specific. "Let's catch A bus. We can talk on THE bus" - ANY bus, but referring to it later with THE.

In oral conversation, with or without "a" or "the", it doesn't bother me, He is a boy, she is a girl. or He is boy, she is girl. Same meaning to me. However, if I am in the teaching position. I will make some correction. "A" and "the", the differences is grammar