The spelling of the word "teastrainer" follows the rules of English phonetics. "Tea" is pronounced /ti:/, "strainer" is pronounced /streɪnər/, and together they form the word /ti:strɛɪnər/. The "ea" in "tea" is a diphthong, which means it represents two vowel sounds (/i/ and /ə/), and the "er" in "strainer" is also a diphthong (/eɪ/ and /ər/). Combining the two diphthongs, the word is spelled with an "ea" followed by "strainer". Overall, proper spelling follows the pronunciation and rules of English phonetics.