"Old classmate" in English can be translated as "former classmate". This term comes from the noun "classmate", which refers to someone who is in the same class as you in school, and the adjective "former", which means "previous" or "having been in the past".

Here are some examples of how you can use this phrase in English:

"I bumped into my former classmate at the mall yesterday. We hadn't seen each other in years!"

"I'm organizing a reunion for all of my former classmates from high school. Would you like to come?"

"I'm so glad I kept in touch with my former classmate. She's been a great support to me over the years."

In Chinese, you can say "老同学" (lǎo tóngxué) to refer to a "former classmate". Here are some examples of how you can use this phrase in Chinese:

"昨天我在商场里碰到了我的老同学，我们好几年没见了！" (Zuótiān wǒ zài shāngchǎng lǐ pèngdào le wǒ de lǎo tóngxué, wǒmen hǎo jǐ nián méi jiàn le!)

"我正在组织一次高中同学聚会，你想来吗？" (Wǒ zhèngzài zǔzhì yī cì gāozhōng tóngxué jùhuì, nǐ xiǎng lái ma?)

"我很高兴和我的老同学保持联系，她这些年来一直是我的支持。" (Wǒ hěn gāoxìng hé wǒ de lǎo tóngxué bǎochí liánxì, tā zhèxiē nián lái yīzhí shì wǒ de zhīchí.)