Grammar exercise 4 of 4 语法练习4
Use 了、是…的、以前 as “Past events in Mandarin Chinese grammar (there's no past tense!)” explains it, through 9 items over 8 of its example sentences: fill the gap where the word was taken out, and put the words of each sentence back in order.
Past events in Mandarin Chinese grammar (there's no past tense!)
9 items. Each one you miss comes back later in the same sitting.