B2 to no longer know what is happening, or not to remember something忘记;跟不上最新的情况
What he was saying was so complicated that I lost track after the first couple of sentences.他说得太复杂,我听了开头的几个句子以后就跟不上了。
I've lost track of the number of times he's asked me to lend him money.我记不起他有多少次问我借钱了。