Grammar exercises: Past events in Mandarin Chinese grammar (there's no past tense!) 语法:了、是…的、以前 练习
The example sentences of the article, a few to an exercise, in the order the article shows them. Each exercise takes the point's word out of a sentence for you to fill back in, then gives you the sentence's words to put in order, and an item you miss comes back later in the same sitting.
4 grammar exercises over 44 items.