1. A vowel followed by one or more consonants. The vowel sound is "closed in" by the consonants, preventing it from being long.
2. The vowel must be short. This is a consequence of the first requirement, as the consonants prevent the vowel from being pronounced long.
For example, the word "cat" has a closed syllable: the vowel "a" is followed by the consonant "t," and the "a" sound is short.