MANDARIN SENTENCE PATTERN
Completed events with 了
Introduce the aspect particle 了 without treating it as a past-tense ending.
Verb + 了 (le) + object
了 after a verb can present an event as completed. It does not conjugate the verb into an English past tense. A sentence can refer to past time without 了.
Sentence-final 了 also has a change-of-state use. Learn these uses in context rather than adding 了 to every sentence about yesterday.
Examples
我买了一本书。
I bought a book.
她喝了一杯茶。
She drank a cup of tea.
我没买书。
I did not buy any books.
Keep in mind
Negating a completed event normally uses 没 and removes the completed-event 了.