What happened for sprint if a member of team finished his task early?

Problem 1

I create a sprint with 8 days.

A member of team finished his tasks after 6 days.

So, What happened for this guy and 2 days for his?!

Beats me. Maybe he goes on vacation, or helps the others with their tasks. Whatever the truth, a good Scrum Team will self-organize their work to best meet their commitments.

2- Problem 2

I want to start 2 story.

One of them takes time about 10 days.

And another of them takes time about 4 days.

How can I manage it?

You don't, but the team does.

What matters is that a team collaborates to provide a Done increment that achieves the Sprint Goal.

Short answer to both of your problems is that the Scrum Team needs to provide solutions to these problems and you should not be dictating how they work. 

Problem 1

I create a sprint with 8 days.

A member of team finished his tasks after 6 days.

So, What happened for this guy and 2 days for his?!

The first part of this that I think is a problem is that you created a sprint.  The Scrum Team should be creating the sprints and planning the work.  They should work together to complete all of the work that is needed in order to achieve the Sprint Goal.  They should not plan 8 hours a day worth of work for every team member.  They should plan work that will provide a valuable increment that satisfies the Sprint Goal.  

As for what that one guy does for 2 days, that is totally up to him and the team.  In all of the teams I have ever worked with, there is usually something that they can do that either helps the rest of the team satisfy the Sprint Goal or helps to refine items that are in the Product Backlog.

2- Problem 2

I want to start 2 story.

One of them takes time about 10 days.

And another of them takes time about 4 days.

How can I manage it?

Again, the first problem I see is that you want to start 2 stories.  Unless you are a developer, that isn't something you decide.  As this section of the Scrum Guide states  the Sprint Backlog is a plan by and for the Developers.  They are the ones that decide what work will be done during the Sprint in order to satisfy the Sprint Goal.  The section of the Scrum Guide that describes the Product Backlog mentions this

Product Backlog items that can be Done by the Scrum Team within one Sprint are deemed ready for selection in a Sprint Planning event. They usually acquire this degree of transparency after refining activities. Product Backlog refinement is the act of breaking down and further defining Product Backlog items into smaller more precise items. This is an ongoing activity to add details, such as a description, order, and size. Attributes often vary with the domain of work.

If your sprints are 8 days, then your team needs to refine the product backlog items into work that can be completed in 8 days or less.