It's a matter of question or statement. The word order is different. When using a modal verb like "can," the action verb always comes after unless it is a question. For example: "I can do it." (The action verb "do" comes after the modal verb "can") In your example: "How can I help you?" (the subject is placed in between the modal verb "can" and the action verb "help," because it is asking a question) "I don't know how I can help you." (This is not a question, so the action verb "help" comes after the modal verb "can." Even though, you use the interrogative word (words used to start question) "how," the word order is not affected, because "how" is used as a pronoun referring back to what the speaker does not know.) Does this make sense? I hope this helps. If you have any further questions, please ask.

This is a very common error, and many of my students find it confusing. "How can I help you?" is question word order. Question word order = (Who/What/Where/When/Why/How) + auxiliary/helping verb + subject + main verb + everything else However, after "I don't know" you use statement word order, not question word order. Statement word order = (Who/What/Where/When/Why/How) + subject + (auxiliary/helping verb) + main verb + everything else Even if it's a question (see below) we use the statement word order. Can you tell me where you live? (NOT Can you tell me where do you live?) But if it's just a question, we say: Where do you live?