hi guys i have a simple question

why when we use . (dot) to execute a shell script in current shell ( . scriptname )we dont need to set executable permission to script
but when we want execute a script like ./script we must set executable perm to the file
??

This doesn not execute the script, it parses the lines in the file (one at the time). This is done in the current shell (the one you parse the file from).
This executes the script. A new (sub)shell is created in which the script is executed.

For parsing you do not need execute permissions, just read permissions.

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Dot '.', in Linux use, can represent several things. In the case of ". script" it is 'sourcing' the script into the current shell. That means it is executed in the current context and any changes to environment variable (PATH, etc) will affect the current session.

Another meaning has to do with file system and path. "." and ".." represent current directory and directory above respectively, when used as path elements. The command ' . ./script ' will source a script in the current folder: useful when the current folder is not in your path and the script might otherwise not be found. The simple "./script" attempts to execute the script in the current folder WITHOUT sourcing it, the more normal case.

There are far better and clearer tutorials and explanations with better examples online, but searching for a period using Google is pretty worthless. Most of the beginning Linux, Unix, and shell (BASH) tutorials cover the subject well, and are easier to find.