I appreciate very much that you have taken in consideration my query.Thank you for your overview of this structure.But you still did not meet my demand. The two sentences below show possibility but why in the first sentence the answer is (can) and the answer of the second sentence is (could) although both sentences indicate the present tense.
1-Driving a car while talking on a mobile phone (can)be dangerous.
2-Without my glasses I can't see what that is on the wall,but it (could)be a spider.
I think that in the first sentence,I am more sure of the result and it can happen in any situation(general situation)but in the second sentence, I am not sure what is on the wall .It is a suggestion.And it happened in a particular situation
Thank u very much Sir.