How does the Sentinel feat work with the Extra Attack class feature?

Question

How does the Sentinel feat work with the Extra Attack feature?

Following this answer about Actions/Reactions etc, it says:

Reactions are actions that you take out of turn (or possibly on your turn) that happen in response to something else.

Can I follow my "Sentinel Attack" with an Extra Attack?

Extra Attack ^{[PHB p. 72]}

Beginning at 5th level, you can attack twice, instead of once, whenever you take the Attack action on your turn. The number of attacks increases to three when you reach 11th level in this class and to four when you reach 20th level in this class.

Sentinel^{[PHB p. 169]}

You have mastered techniques to take advantage of every drop in any enemy’s guard, gaining the following benefits: [...]

• When a creature within 5 feet of you makes an attack against a target other than you (and that target doesn’t have this feat), you can use your reaction to make a melee weapon attack against the attacking creature.

Answer 1

You already bolded the answer.

Beginning at 5th level, you can attack twice, instead of once, whenever you take the Attack action on your turn.

If you're not taking the Attack action on your turn, then you can't use Extra Attack. Taking the Attack action doesn't just mean attacking - it's a specific action you can perform, listed under Actions in Combat.

Answer 2

Sentinel allows you to "use your reaction to make a melee weapon attack".

Extra Attack triggers when you "take the Attack action".

Note the capitalisation of Attack action. That's a clue that it's not just "an attack". It's a specific action. A melee weapon attack is something you can do with the Attack action, but it is not the Attack action itself.

If you have Extra Attack, the Attack action allows you to attack twice - that's two melee attacks but only one Attack action.

Answer 3

No you can't, because extra attack works only on your turn:

Beginning at 5th level, you can attack twice, instead of once, whenever you take the Attack action on your turn.

And the other creature attack on his turn:

When a creature within 5 feet of you makes an attack against a target other than you (and that target doesn’t have this feat), you can use your reaction to make a melee weapon attack against the attacking creature.

Answer 4

On your character's turn, you can move a distance up to your speed, and take an action. Additionally, a feature may provide you with a bonus action.

The options for your action include:

1. Attack action
2. Cast a Spell action
3. Dash action
4. Disengage action
5. Dodge action
6. Help action
7. Hide action
8. Ready action
9. Search action
10. Use an Object action

Your "Extra Attack" feature interacts with choosing to take the bolded "Attack action."

Outside of your turn, you also have a "reaction" that can be triggered by listed conditions. Sentinel specifically triggers when a creature within 5 feet of you attacks some other than you, giving you the ability to make a melee attack against them. You aren't taking the Attack action, just making a melee attack.