Here's why:
* Short "e" sounds: These sounds are often spelled with "e" followed by a consonant. Think of words like "bed," "pen," and "set."
* Long "e" sounds: These sounds are often spelled with "e" at the end of a word, followed by a silent "e," like "cake" or "these." Or they can be spelled with "ee," "ea," or "ey" as in "tree," "meat," or "they."
Since "breath" has a short "e" sound, similar to the sound in "bed," we know the "e" is short.