Language educators find two kinds of errors in the interlanguages of language learners: developmental and interference. While developmental errors reflect a normal pattern of development common among all language learners, interference errors are caused by the learners' native languages. This paper deals with a number of die-hard types of interference errors found in English written by Vietnamese speakers. The most persistent errors involve the handing of the linking verb "to be" before adjectives; indefinite and definite articles; complicated verb tenses; subject pronoun and object pronouns; and complex sentences introduced by subordinate conjunctions. It is recommended that these errors be politely called to the speakers' attention and that practice and monitoring are the most effective ways to correct these interference errors. (KFT)