You're visiting a couple who you haven't seen in a few years. You have just finished greeting each other and are now you want to start to catch up with them. You ask:
So how've you been?
This is a good general conversation starter. It's often used right after engaging in a little small talk, to transition into more serious conversation. One of the purposes of using this word at the beginning is that it announces that you are going to bring up a topic but gives the listener time to prepare before you ask a question.
So, I heard you're leaving us.
So, let's talk about what we need to do to prepare for this conference.
In the example above, saying "So" shows that you want to shift from general greetings to discussing more details about each others' lives.
When someone asks, "How have you been?" you should answer with more than just a short word like "Good" or "OK". You should respond by giving news about how your life has been lately. For example, you can answer:
Pretty good. Work's been busy. I've been working non-stop lately.
"How've" is short for "how have". It's pronounced this way in casual conversation.
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