Question about set variable and echo it.

Hi there, I have a question about set variable in a-shell.
If I type `a=1`, it will rise an error "a=1 command not found". The only way can do it is `export a=1`, don't know why.

And another question is quote marks. ```
echo `date`
```, ``` echo "`date`"` ``` both does not work as excepted.
Can someone tell me how to solve this issue