A useful or an useful?
]
Answer: A useful.
The rule for putting an in front of unique or useful:
Although U is a vowel, the pronunciation of useful begins with a Y sound (yoos-ful) which is a consonant sound.
Wrong: I found your post an useful read.
Right: I found your post a useful read.
unique [yoo-neek]
useful [yoos-ful]
usual [yoo-zyoo-l]
In the case of an before a word that begins with u, let your ears guide you:
a useful device, but an unusual device
a unique feature, but an ultra-interesting feature
a unique element, but an unknown element
a uniform-based school, but an unconventional school
a useful paper, but an undervalued paper
Good luck with your writing.
jt