The copula "be" poses problems for learners at early stages of language learning. It functions as both an auxiliary verb and a copula. As a copula, it links subjects to predicate adjectives or nouns. As an auxiliary, it indicates progressive aspect. Unlike other verbs like "walk", the copula "be" has more inflected forms across tenses and subjects. It also behaves differently syntactically, requiring no auxiliary for questions or negatives. Finally, some languages lack a copula, which can pose difficulties for learners.