忘记 (wàngjì) is the standard HSK3 verb for 'to forget' — it's used in both speech and writing and can take a noun or a verb object. Compared to the HSK2 忘 (wàng), which is shorter and more colloquial (often used alone or in fixed phrases like 别忘了), 忘记 sounds slightly more formal and complete. 遗忘 (yíwàng) is a literary/formal word for forgetting abstract things (e.g., history, memories) and is rarely used in everyday conversation.