You're right about two out of the three sentences. 1. "How long did David work for them?" would mean that David worked for them previously but doesn't work for them any more. Yes, exactly. You used the past simple to refer to a FINISHED ( or 'closed') period of time. 3. Present perfect continuous: "How long has been David working for them?" would mean that David works for them at the time of speaking. Yes, that's right. It means, for example, that David took up a post with a certain company ten years ago and he is still there. In this case, you would ask "How long has David been working for them?", and you might get the reply "He's been working for them for 10 years" ( or "...since 2008"). The one which you misinterpreted was number 2. In fact, I don't understand what you mean by "working generally for them" or "how long does he do it" . This doesn't make much sense in English. Sorry. So what does "How long has David worked for them?" mean? As Augusto has explained, it actually means exactly the same as "How long has David been working for them?". They are both present perfect questions, and both refer to exactly the same UNFINISHED ( or 'open') period of time - in other words, the period of his employment which is still current. There is no difference in meaning between saying "He's worked for them for 10 years" and "He's been working for them for 10 years" : they both mean that he's still working there now. This same overlap of meaning occurs with the verb 'live', and sometimes the verb 'study', but not usually with any other verbs. Let me know if you'd like me to explain this further. We're running out of space on this answer!

Both present perfect and present perfect continuous have the same meaning when you use how long. How long have you studied English? How long have you been studying English? Both sentences are asking information that started in the past and is still happening in the present.

Hi, You got the use of how long+simple past right! As far as how long+ present perfect and present perfect continuous is concerned you use both tenses to talk about an action that started in the past and continues up to the present. For example : 'How long have you worked as self-employed?/How long have you been working as self-employed?' The meaning is identical, because it means that you are still working as a self-employed. Watch out you can't use present perfect continuous with stative verbs such as like, think (to express opinions), hate, own, have (for expressing possession), ecc....!With stative verbs you can only use present perfect