This paper discusses what the basic principle of Korean consonantal assimilation and tensification is and how to teach the two processes to learners. Since some of the Korean phonological changes including these two processes are complicated and cannot easily be seen in other languages, students may face difficulty in learning such processes. It has been claimed that these processes are due to the "principle of economy" in pronunciation. That is, certain consonant clusters or consonantal sequences that are difficult to produce undergo changes in a way to make them easy to pronounce. In this paper I argue against such a claim by adopting the principle of English, which has two different types of consonant clusters, namely "word-initial and word-final clusters". The phonotactic constraints of the word-final clusters in this language, which are opposite to the case of word-initial clusters, have the ascending structure in the consonantal strength between the two consonants. We see that the two consonants in sequence in Korean have almost the same structure. Unlike English, these constraints must be obeyed whether words are simple or complex in Korean. The two consonants that are not kept up to these constraints by the morphological process undergo phonological process. This is the principle of consonantal assimilation and tensification in Korean, and thus, teachers (and also possibly learners) of Korean should recognize the principle to understand the processes correctly.