Here's how to tell:
* Short "a" sound: Think of the sound in words like "cat," "hat," "man," or "ran." It's a quick, clipped sound.
* Long "a" sound: Think of the sound in words like "cake," "name," "rain," or "say." It's a drawn-out, vowel sound.
Since "lamp" sounds more like "cat" than "cake," it has a short "a" sound.