Vowels are longer when followed by a voiced consonant. They are shorter when followed by a voiceless consonant. Even short vowels like /i/, /ɛ/, /ɘ/, and /ʊ/ are prolonged when followed by a voiced consonant.

Warning: Common Mistake

When you lengthen a vowel, make sure that you do not change the sound of the vowel.
For example, when you say hid make sure that it doesn’t sound like heed.