Sidebar if condition

Is there a way to place an if condition in the sidebar.php to display certain elements depending on what page, or category is being displayed?

For example I want the Recent posts to show up on sub pages, but not on the index.php.

Also I want the subcategories to be seen only on the category-4 page but not on other.

Please let me know if this is possible.

Also, I’m a newbie, when it comes to php, so please keep that in mind and try and simplify the explanation.

Thanks.