For example, after clicking open declaration, is there a shortkey which let me back to the previous tab?
If you are talking about switching between open files, you can do this using Ctrl+PgUp and Ctrl+PgDn
If it is about navigating back & forth to the last modification, use Alt+⇨ (right arrow) and Alt+⇦(left arrow)
Ctrl + Shift + L
to view all shortcuts and from them you can have your previous tab shortcut.
By next tab, you probably mean the "Previous File".
By default switching between open files (called "Editors" in eclipse parlance) can be done by Ctrl+F6 and Ctrl+Shift+F6
This is configurable, so if you find this combination inconvenient (like me) you can change it in the Preferences: General -> Keys
. Look for Next Editor
and Previous Editor
(I have mapped them to Ctrl+Tab and Ctrl+Shift+Tab.)
Use Alt + LeftArrow to go back.