Difficult question, as answers will certainly vary. If you've studied a foreign language yourself, you can rely on that experience. For English: subject-verb agreement; at least 250 common, everyday verbs; thorough understanding of negative and interrogative formations in the simple present; prepositions (crazy things, but one must begin at some time perhaps studied in "sets" with a few examples of each); basic word order S+V+O (difficult for speakers of language for whom it may be even the reverse); for speakers of Asian languages they need to start very early in learning to "close" syllables that have a final consonant sound - very difficult when leaving a language with open syllables that all end in a vowel sound, and are often mono-syllabic.
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