Short vowels are single vowels that are followed by one or more consonants within the same syllable.
Single vowels that are followed by _ch_ but not within the same syllable (in the following one) are also considered short vowels. This is a nasty little exception hardly worth mentioning, since so few words face it. Nonetheless it must be mentioned.
When a word ends in a consonant, it is easy to see that the single vowel preceding this consonant is a short vowel: the word fregat (frigate) ends in a consonant (t) and is preceded by a single vowel (a), which is thus a short vowel.
Figuring out whether a vowel is short or long
If the single vowel is somewhere _in the middle_ of a word, you need to divide the word into syllables to see whether the vowel sits at the end of the syllable (long vowel)
or whether it is followed by a consonant within the same syllable (short vowel).
What about the following vowels? Are they long or short?
afmeting
af - me - ting
The letter _a_ is followed by the letter _f_, and is thus a short vowel.
The letter _e_ is an open vowel, which makes it a long vowel. The letter _i_ is followed by the consonants _ng_ and thus a short vowel.