How can i send message to a player during an event?

well how can I?

It depends on what event you're going to use.

For example, Entity events:
$event->getEntity()->sendMessage("your message here");

For Player event:
$event->getPlayer()->sendMessage("your message here");

There is no sendMessage() method in the Entity class
>.< check instanceof Player.

Or well, it should depend on who you are sending to. After getting the player object, ->sendMessage("message") it.

There are different methods to get different players in different cases. Read the docs for details.

uhh the $event here is the actual event's variable? like

PHP:

public function whatever(whateverevent $wee){
}

the $wee there?

Yes, but different events have different methods.

You are a good observer. Continue studying other plugins and you gonna make successful plugins?

can you mention some events like that?

Refer to https://github.com/PocketMine/PocketMine-MP/tree/master/src/pocketmine/event