Develop Reference

should I use fragment or viewpager for my app

Demo1
Demo2
when users click the button to answer the questions for the TextView on the top, the TextView will change the questions depends on the user's answer.
(I think this part won't be hard for me)
But what should I do after user click the button, the buttons will change as well, please see the demo picture.
are there any skills I should study first? like framelayout?
Thank you for your answers.

It really depends on specific needs, but generally speaking, google guidelines are toward the use of fragments.
I'm developing Android apps since just few months and I'm using the single activity/multiple fragments approach. You have to learn how to use fragments, e.g. their lifecycle compared to the attached activity or how to handle events among fragments (EventBus can help you for such task).
You can find more discussions about this approach here
Cheers.

you can create a N fragement with your nuber Button and you set your buttons in this fragement (Demo 1)
but in click a one button you can replace a Specific Fragment (Demo 2) in your framelayout be placed in your Activity

Create android actvity with divider between 2 views

I don't know exactly how to describe it, but I want to create something like this:
Basically it is an activity with 2 views, and you can reveal/hide more of one view by sliding the "slider" up and down.
I have an idea of how I would implement this on my own, but I am wondering if there is some built in widget for this. The inspiration came from my Sony Experia, where I swipe down to reveal a drawer of notifications.
I thought maybe it was the SlidingDrawer, but that has depreciated in API 17.

There is no Splitpane or something similar in the current Android API (19 at writing).
But there are existing user created views you could use. For Example this one.

fixed tab view to display 3-4 screens in android

I want a tab view activity which will display 3-4 screens in it. I don't care if tabs are fixed or can swipe but I want it to support atleast API 8 or min. a rough overview you can get by below image.
Please let me know what should I use and is there any build-in API or something that can help.
Thanks for your help.
Regards,
Sourabh

Navigating using tabs is easily implemented using TabHost. There is one main Activity which lists all the tabhosts, and they define the other screens. This is explained very well in this tutorial. http://www.mkyong.com/android/android-tablayout-example/
The code for each screen is written on a seperate Activity.
Hope this answers your question.

android: How can I implement first time tutorial like Go Launcher in my app?

Go Launcher have a nice first-time tutorial. it is very similar to Stock ICS first time run. I want to learn how to make the similar tutorial display at the first launch of my app. How can I implement this Transparent view (which interacts with screen objects) in my android app?

I was trying to do something similar using a transparent activity with a viewpager inside it. I wanted static 'tutorial' images that users could swipe through.
One of the answers to my question mentioned an interesting library (called ShowcaseView). It seems like it may be a good fit for your requirements as well.
https://github.com/Espiandev/ShowcaseView

You can either use a FrameLayout or RelativeLayout as root of your Activity/Fragment, and put your "first time" View upon content with a (almost) transparent background, or use another Activity/Fragment themed with a transluscent window. (like this)
I'd rather use an Activity/Fragment, but this is up to you!

I wrote little library That contains a simple component for making ​​a app tour. It`s very restricted to my case but maybe could be your case . Single LessonCardView Showed onstartactivity for the first time or on button click Any criticism ,help or advice will be appreciated. thanks https://github.com/dnocode/DnoLib

You can implement a first-time user guide with the Spotlight library by TakuSemba.
It is similar to the one shared above and it is still well-maintained (as of 2021).
https://github.com/TakuSemba/Spotlight

Android - Multiple activities under a single tab, Fragments

After scanning all the related questions regarding multiple activities under one tab, I found out that they all refer to using ActivityGroup which is deprecated.
Basically I have 2 tabs, the first one contains a main activity with several buttons and when button is clicked 1 for instance a new activity should open under the same tab, clicking back should bring me to the main view under the first tab. The second tab contains only one activity.
Tried inflating a new view for the sub-activities:
FrameLayout frameLayout = tabHost.getTabContentView();
frameLayout.removeAllViews();
View view = getLayoutInflater().inflate(layoutID, tabHost, false);
frameLayout.addView(view);
But I only get the view, I cannot handle it like an activity; meaning adding buttons or listeners, it's only a view.
Google suggests using fragments instead, but I am having the hard time implementing this. Maybe replacing the activities with fragments could be the solution.
I could really use a good tab\fragments example right about now...
Any suggestions? Thanks in advance.

Yes, the solution to your problem is using Android Fragments.
I understand that you have trouble figuring out what to do. It's everybody's difficulty actually. However, with enough patience to google out things, you will realize it's actually not that difficult.
What I will be telling is basically some tips on how to change your code to fragments. From your current code, there's actually a major change you need to do but it's worth it.
What you are doing is actually you're making a dynamic UI. You have chunks of fragments which eventually can have different listeners which you should later on defined. Here's a sample demo you can actually wok on.
If you want a detailed tutorial, I followed this one. And, yes, it works!
Have fun learning Android and don't forget Android is directly linked to Google. And you can easily google things and ask specific questions here should that question was never asked before.