Completion 了: V + 了 + amount + object
动作的态 / aspect (完成)
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
Subject + Verb + 了 + quantity + Noun / 没 + Verb
了 after the verb marks an action as completed. When the object carries a quantity or detail, the sentence can end there: 我买了两本书.
Without a quantity, ✗ 我买了书 sounds unfinished: add a quantity, a final 了 (我买书了), or a follow-up (我买了书就回家了).
Negative: 没(有) + V, no 了: 我没买.
Examples
去年我去了三个国家。
Qùnián wǒ qùle sān ge guójiā.Last year I went to three countries.
我在那儿拍了很多照片。
Wǒ zài nàr pāile hěn duō zhàopiàn.I took lots of photos there.
我下了课就去找你。
Wǒ xiàle kè jiù qù zhǎo nǐ.I'll come and find you after class.
昨天我没去上班。
Zuótiān wǒ méi qù shàngbān.I didn't go to work yesterday.
Common mistakes
✗ 我没去了 → ✓ 我没去.
✗ 我昨天买书了三本 → ✓ 我昨天买了三本书.
了 isn't a past tense: past habits don't take it: ✓ 我以前常常去那儿.