This article explains how to tag a friend when you comment on a Facebook post. Once nominated, the user in question will be notified.
Steps
Method 1 of 2: iPhone / Android

Step 1. Open the Facebook application
The icon looks like a white F on a blue background.
If you are prompted to log in, enter your username and password, then click "Log In"

Step 2. Posts will appear on the timeline or home page of a particular friend
Scroll down the page and see if you want to comment on one.

Step 3. Tap Comment
It is located below the post.

Step 4. Type @ + your friend's name
As you type, a drop-down menu will appear with the various search results.
If the friend's name appears on the list before you finish typing, tap the result to automatically tag it

Step 5. Finish writing your comment and tap Post
The comment will appear in the appropriate section of the post and your friend will receive a notification as soon as they open Facebook.
Method 2 of 2: Desktop

Step 1. Open Facebook
If you are prompted to log in, enter your username and password, then click "Log In"

Step 2. Several posts will appear on the timeline or page of a particular friend
Scroll down the page until you find a post you want to comment on.

Step 3. Click on the comment box
It is located at the bottom of the post comments and inside you will read "Write a comment …".

Step 4. Type @ + your friend's name
As you write, a list with various search results will appear.
If your friend's name appears in the list before you finish typing, click on the result to automatically tag it

Step 5. Finish writing your comment and press the Enter key
It will appear in the comments section and your friend will be notified as soon as they connect to Facebook.