Reader Alan mentioned the pronunciation of aftermath in a comment to a recent post, in particular the pronunciation of the second a. I cannot use myself as an example of the standard English, or RP, pronunciation of this vowel as a long ah sound (/ɑː/ is the phonetic symbol), as I use a short a in words such as grass and path.
The way one pronounces the vowel a is the main difference between northern and southern English speech. I discovered, when looking up what the pronunciation of aftermath should be, that there is an interesting history to the pronunciation of an English a, and things are often not as clear-cut as some would have us believe.
Until around the 1600s everyone, no matter where they lived, pronounced a the short way, as in trap. But in the 17th century it became fashionable in London to lengthen the a sound before the sounds s, f and th (so-called 'bath' types of words). This pronunciation was then exaggerated, making the a sound even longer. Then the long a began to appear before an m or n if they were followed by another consonant (grant, sample, for instance).
There are by no means hard and fast rules about which words are pronounced with a long a and which with a short a. Although I don't say 'graahss' or 'aahsk', I have always used a long a sound in the words plaster and master. Most people have anomalies like this. The pronunciation of plastic and photograph often don't obey the rules (and aftermath is probably one of these too). Wikipedia says that generally, the more common the word, the more likely it is to have a long a, and monosyllabic words are more likely to have it than polysyllabic words.
The British Library has a very interesting and not at all technical article on this subject, which I recommend if you are interested in more details.