I buy it [ ] a good price,3yuan.A in B at C onI buy it [ ] a good price,3yuan.A in B at C on为什么用at 是固定搭配吗?He left sz today.today 为什么用过去时 不是现在时吗

at a good price-就是固定搭配
如：
Where can I get rechargeable batteries with recharger at a good price?
The shares were bought at a good price.
If they can get it at a good price they'll take it.
left---肯定已经离开了.事情已发生的话就要用过去式,哪怕是一分钟之前已经算是过去.