Ah I love articles. A/An is used when you are introducing a noun for the first time to a listener who has no knowledge of that noun. For example, if you want to introduce a child to a car, you must say- This is A car. "An" is used before introducing nouns starting with a vowel, such as an elephant or an orange. This is however not absolute, such as in the case of A European. Now, when you are talking to the same child about the same car after introducing it, you must use THE. Example - This is A car. THE car is red. "The" is also used to talk about things that are very unique, like THE Sun or THE Moon and ever names or rivers or mountains. We do not use the before the names of countries, however. "At" and "in" are propositions of time and place. Let's talk about place first: At is used when talking about a general location. For example: He's at the movies or She's at her friend's house. "In" is a bit more specific in regards to location. Example: I am IN the shower or He's IN that room. Now lets talk about "at" and "in" as prepositions of time. In this case, things are a bit reversed. "In" is used to talk about less specified time, like an entire year or century or decade or even a phase of the day, like morning, evening, afternoon, etc. Example: I was born in 1993 or He's coming home in the evening. "At" however is USUALLY used for very specific time, like at 10AM in the morning. I hope this helps.

Thanks, Odio!)

this vedeo , can help u more / https://www.youtube.com/watch?v=4H7VpBwYPOE