Learning the sounds alphabet letters make, as opposed to simply their "name", is the basis for a child learning to read. It's all well and good to point to a letter, ask "Which letter is this?" and then the child has no idea letters make sounds. Children seem to understand when told that letters have jobs. Their job is to make a sound. Some letters even have two jobs; they make more than one sound.
If a child is in school and falling behind reading, this is the most crucial base to cover. Have a stack of picture free alphabet cards made with index cards and a marker. Show the child a card and ask what is the letter's "name" then what sound this letter makes. If the answer is correct, start a "correct" pile. Proceed through the entire alphabet putting incorrectly answered cards in another pile AFTER naming/sounding out the letter for the child.
Vowels can be tricky. Knowing correct vowel pronunciation is vital. If not learned correctly, it will be a huge stumbling block down the road when bigger and bigger words are decoded, IE sounded out.
JUST DON'T MAKE A BIG DEAL ABOUT IT
When testing a child, often times parents are quick to say their child has known the alphabet since the age of two. I am more interested if they know correctly the sounds each and every letter in the alphabet makes. Incorrect letter sounds are the pot holes which derail a child's reading skills. It has flabbergasted more parents their child did not correctly know the alphabet sounds.
THIS IS STEP ONE
Go over and over the alphabet until the child learns all letters with their correct sounds.
It can be done in as little as five minutes a day.
Don't get exasperated. The more exasperated you get, the longer it will take to learn.
If necessary, concentrate on three letters at a time, then go on to the next three.
If you have a young child just learning the alphabet, always always always, say the sound.
"This letter's name is A. His job is to make a special sound. The sound is AAAAAAA (short A sound).
Can you say AAAAA?"
This is called mapping print to speech. Without this step we do not learn to read.