can we use could and can in one sentence

Absolutely. We can combine can and could when speaking about ability, possibility I can speak English and Hindi, but when I was a child I could  speak Punjabi as well. Now we can easily connect with anyone via internet, but in the past we definitely couldn't.

Yes , I can use can and could in the same sentence but here "can" means a cylindrical vessel for liquid. Ex..That can is very cheap so I could buy it.

yes, we can use.