ndeed, the rule is that c followed by a, o, or u is hard (/k/), whereas it is soft (/s/) when followed by the vowels i or e.; however, the rule, as most "rules" in English, suffer not infrequent exceptions. Consider, for instance, the word cello (from the Italian) which is pronounced chelo, and the words cellophane (from the French) and its popular abbreviated form cello which are pronounced selofane and selo; or the word cist which can be pronounced sist or kist--as strikes your fancy (Kenyon and Knott, 1953). The EFL teacher may have a hard time explaining these ambiguities, nuances, and inconsistencies to Japanese students.
Consonants are often classified in terms of their anatomical or physiological aspects, viz. the state of the glottis (whether there is voice or vibration in the larynx), the place of articulation (that part of the vocal apparatus with which the sound is most closely associated), and the manner of articulation (how the sound is produced). In this perspective, the sound /k/, for example, can be described as a "voiceless velar plosive", where "voiceless" refers to the state of the glottis; "velar", to the velum as the place of articulation; and "plosive" to the manner of articulation (the release of a blocked stream of air).
It is noteworthy that in English the distinction between vowel and consonant sounds and symbols is by no means always straightforward. The sounds and symbols j, v, w, and y, in particular, function as vowels as well as consonants. Compare, for instance, suite/sweet and laniard/lanyard. Phonetic analysis may class such letters as semi-consonants or semi-vowels, depending on whether one believes that a glass is either half-empty or half-full.
Whether there is voice or vibration in the larynx, i.e. the state of the glottis, is one of the anatomical features which is instrumental in forming consonants. The glottis is the space in the larynx between the vocal cords or folds. When the vocal c...