On this page, I will write down over the time useful Illustrator tricks.

Command + 2: Lock

Command + 3: Hide Object

Command + 7: Layer mask

Ctrl + D: Repeat the last effect
(While in Rotate tool) Opt + Click: Set the anchor point

E: Free Transform
Especially when you want to rotate texts

(To avoid two different paths from connecting in Pen Tool)
First click empty space and keep holding. While holding the click, press Space and move around cursor to place an anchor, which won’t connect with another path.

Press down Shift and Delete Anchor Point Tool to delete an anchor and keep the curve.
Press Option to toggle Add Anchor Point Tool and Delete Anchor Point Tool