This pattern is used when you want someone to tell you what to do. It's like saying 'What should I do?' but more specific: you can ask about a particular action by putting that action in the た-form. For example, if you're lost, you might ask 'どうしたらいいですか' (What should I do?). But you can also ask about a specific verb: '何を食べたらいいですか' (What should I eat?). The nuance is that you're seeking guidance or a recommendation.

It's a polite and common way to ask for instructions, suitable for speaking to teachers, seniors, or strangers. You can also use it with nouns or な-adjectives by adding だったら: for example, if you want to know what kind of gift is good, you might ask 'どんなプレゼントだったらいいですか'. But for verbs, the た-form is essential.

Remember that the た-form is the same as the past tense form, so '行く' becomes '行った', and you say '行ったらいいですか'. This is not asking about the past; it's a set pattern for hypothetical advice.