Why we say "an" H.B. pencil, not "a"?

Because when speaking, the letter H is pronounced "aitch", which commences with a vowel sound. Other consonant letters whose pronunciations start with vowels are

F ('eff')
L ('ell')
M ('emm')
N ('enn')
R ('arr')
S ('ess')
X ('eks')

Conversely U is treated as if it starts with a consonant because it is pronounced 'yoo'.

an A
a B
a C
a D
an E
an F
a G
an H
an I
a J
a K
an L
an M
an N
an O
a P
a Q
an R
an S
a T
a U
a V
a W
an X
a Y
a Z

See TYN's post. In English, we use "an" before vowel sounds. So we say A hard problem, because the "h" is pronounced; but we say AN honorarium because that "h" is not pronounced.

many thanks!

Likewise hour, heir. In British pronunciation historic, horrific, and hotel used to drop the initial 'h' but this is considered somewhat old-fashioned or affected nowadays (I myself drop those three 'h' sounds, and have always done so). In dialect speech e.g. Cockney, people will say e.g. 'An 'orrible way to die'.

Also the definite article 'the' is pronounced 'thee' before a vowel and 'thuh' before a consonant e.g. thee earth, thuh moon, thuh USA, thee FBI, thuh BBC, thuh KLF (a '90s pop group).

I use "an" before history, historic, etc. I don't care if other people like it, and rather hope that they don't.

In certain areas of New England they will have "A" herb garden

whereas here in PA we have "An" herb garden

There are also quite a few Brits who have the annoying habit of pronouncing the letter aitch, haitch.

If such a person is asking for an aitch bee pencil, then they would (supposedly) ask for a haitch bee.


Grrrrr.

I never promised you a herb garden...

But I already have an erb garden.
Also, a weed garden.

This is all interesting to me, making me realize how tricky it is to learn english. By now I don't remember learning the rules for these differences, I just automatically say them the way you describe, sans thought re the wording.

Here in Britain many people imagine that all North Americans say 'erb, usually rhotically, but this is not so, as you say. I heard someone from Baltimore say it almost in a Cockney fashion (i.e. silent R), but pronouncing the H.

we pronounce the H and are always corrected by people who should be playing in their own gardens and should just leave me the hell alone

Thing about it when we say an Hb we say aech b
This also occurs in hotel we say an oetel