...until the day when he was taken up, after he had given commands through the Holy Spirit to the apostles whom he had chosen. (Acts 1.2)How was Jesus teaching them while He was on the earth after His resurrection? Through the Holy Spirit. I'm not exactly sure what that means, but I have a working idea.
I think the Holy Spirit used the words of the Scripture to show them throughout the Bible how God had predicted this kind of Messiah, a suffering yet victorious Savior. My wife illustrates it this way: the Spirit's voice in our lives sounds a lot like the voice of our mom or dad telling us to look both ways before crossing the street. And my guess is they had a lot of those moments when they heard the voice of Jesus in their head, listening to the words He had said.
Jesus hadn't left yet, though. So when they had a moment with Him, He was helping them understand not only what was said but how it was they came to hear it.
And that's important.
Because the Spirit still speaks to the people of Jesus. He always points to Jesus and never contradicts His Word. But He does still speak. That's what makes it a living, breathing relationship.