According to an online dictionary (spanishdict.com), one can say "I'm sorry" in three ways:
- "perdón" to apologize (Perdón por...)
- "lo siento" in more formal occasions (Lo siento mucho por su pérdida)
- "disculpa" which is also more formal than "perdón".
If I want to say "I'm sorry, but I can't help you", which verb would be best to use?