"Can certainly relate" emphasizes the speaker's strong agreement or understanding. It suggests that they fully grasp the situation and can empathize deeply.
"Can relate" is a more general statement of understanding or empathy. It doesn't necessarily imply as strong a connection as "can certainly relate."
Here are some examples:
* "Can certainly relate": "I can certainly relate to feeling stressed before a big presentation." (This implies a strong understanding and shared experience of stress.)
* "Can relate": "I can relate to wanting to travel the world." (This implies a general understanding of the desire to travel, but doesn't necessarily mean the speaker has a strong connection to it.)
Ultimately, the best choice depends on the context and the level of emphasis you want to convey.