Вкратце: если ударение падает на первый слог и вы как будто "выдыхаете" первую букву - то надо использовать "a". Например: horrible, happy, hospital, home, history.

А если ударение падает не на первый слог и "h" не слышится - то "an". Например: historical, habitual

Просто произнесите вслух это слово и сразу поймете, какой артикль ставить перед ним.

Фрагмент статьи на эту тему на английском:

"It all comes down to pronunciation. There are some h-words no one would ever dream of putting an before, like horrible, happy, hospital or home.

But while you might use an before historical, you wouldn't before history. Similarly, you may before habitual but would never before habit. So, what the heck?

It comes down to which syllable of the word is stressed. Try saying these words aloud and you’ll find that for those where you land more heavily on the first syllable, you fully pronounce (aspirate) the h. For these you would automatically use a. But where the first syllable is unaccented, the h tends to soften, or is lost altogether, making an seem natural – at least to some ears."