So far, we’ve seen that could is often used as the past tense of can. Other important meanings and uses include the following.
•Use could (not can) to refer to conditional situations, in which something has to happen or be the case in order for someone to be able to do something or for something else to occur:
We could buy a new sofa if we stop eating takeaway meals every night.
I wonder why can cannot be used here. I would think if can were used, it would suggest capability or opportunity to do this in the situation, which would be quite reasonable.
Are there any other examples which can justify this usage?
Please help to clarify