"A" and "an" are used for singular nouns like "a telephone" or "an apple". Usually words that start with a consonant use "a" and words that start with a vowel us "an" but there are exceptions. "The" is used for specific items like "hand me THE telephone (as in that one)" or "I handed him THE apple". At is used for places to tell where something is. An example paragraph would be: I bought AN apple AT THE grocery store. When I got home I put THE apple on the counter. Later I took THE apple with me to work and ate it AT my desk along with A sandwich.

Here are some examples when using 'a, an, the at' : A dog. That is a dog. You are a nice person. An apple. Where is the apple at? Do you have an apple? "The dog over there on the rock around the pond is eating an apple beside a tree trunk's base. "