'Could' could refer to the present/future when referring to possibility according to definition#2 at http://www.ldoceonline.com/dictionary/could
Student or Learner
It is there. (simple presnt tense)
It was there. (simple past tense)
I can go.( simple present tense)
I could go.( can is past of could, so is it present tense?)
If yes, Could you please tell me the reason?
Syntactically, the modal auxiliary "can" has two tenses: past (could) and present (can). So your example "I could go" is a past tense sentence. But semantically "could" does not only refer to past time. In fact it has three main uses: past time (Ed could be a real nuisance at times); backshift (My teacher told me I could pass the exam if I studied really hard); modal remoteness especially in conditionals (I could do it if they paid me enough).
One more doubt.
I could go wrong in my spoken English.
It is past tense but it refers to present and future. Right??
Yes, in the right context.