Ability to Let Members Subscribe to Posts

Is it possible to create an ability to the forum where members can subscribe to posts which they feel helpful to them? If the request is possible, how do I do that??
Thanks in advance.

Simply check "Watch this topic for replies" in the bottom right of each topic

is any mode to make automatic subcribe to all posts that a user has replied?

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Yes, they can. If a user wants that to happen, they can go to their profile, click on the preferences tab, then on the Always notify me of replies option set it to yes and hit save. This will make that check box, mentioned above, checked automatically. They can uncheck it still, but the default is to check it.

can I(admin) make it for all users ???

You can't force it, they can always change it back. You would have to go and update each users' profile, one at a time, to do that. However, for each post they make, they can uncheck it, or they could go into their profile and just turn it off.