Here's how to tell:
* Short A sound: Think of the sound in words like "cat," "hat," "man," and "ran." It's a quick, closed-off sound.
* Long A sound: Think of the sound in words like "cake," "hate," "name," and "rain." It's a drawn-out, open sound.
Since "ant" sounds more like "cat" than "cake", it has a short A sound.