You can use my pen
You could use my pen
What's the difference? Can we use could for advice?
If someone asks you if you can use your pen, you would probably reply
Yes, you can use my pen
Yes, you could use my pen
As using "could" implies that you might or might not give the person your pen, while "can" implies that they can use your pen
Definition of could for reference:
could: used to indicate possibility
It is certainly possible to use could for a suggestion, which is a kind of advice.
You could take the train.
You could use a hammer.
In the case of you could use my pen it could also be about giving permission, and it might be ambiguous whether it is permission or suggestion: usually the tone of voice will make that clear.