Grammar exercise 1 of 4 语法练习1
Use 了、是…的、以前 as “Past events in Mandarin Chinese grammar (there's no past tense!)” explains it, through 13 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!)
13 items. Each one you miss comes back later in the same sitting.