Why is the an bhfuil construction not used here?
Because the verb is scríobh, not bí.
Scríobhann an scríbhneoir. - "the writer writes."
An scríobhann an scríbhneoir? - "does the writer write?"
An bhfuil an scríbhneoir ag scríobh? - "Is the writer writing?"
Thanks for that.
Why does the question begin with the word 'an?'
It's the present tense question particle - it signifies that what follows is a question. It shouldn't be confused with the definite article an.
Go raibh maith agat :)
Can someone remind me why it isn't 'an tscríobhann...' ?
The interrogative particle an causes Eclipsis and you can't eclipse s.
s takes a t-prefix in certain cases, but that's not eclipsis.