Why do we say "it's too hard a task" and not "it's a too hard task"?
Is there a rule for that?
In most cases, the article is the first word of its noun phrase, preceding all other adjectives and modifiers.
The little old red bag held a very big surprise.
There are a few exceptions, however:
- Certain determiners, such as all, both, half, double, precede the definite article when used in combination (all the team, both the girls, half the time, double the amount).
- The determiner such and exclamative what precede the indefinite article (such an idiot, what a day!).
- Adjectives qualified by too, so, as and how generally precede the indefinite article: too great a loss, so hard a problem, as delicious an apple as I have ever tasted, I know how pretty a girl she is.
- When adjectives are qualified by quite (particularly when it means "fairly"), the word quite (but not the adjective itself) often precedes the indefinite article: quite a long letter.