Narset, Enlightened Master — ruling 1

The cards are exiled face up. Casting the noncreature cards exiled this way follows the normal rules for casting those cards. You must follow all applicable timing rules. For example, if one of the exiled cards is a sorcery card, you can cast it only during your main phase while the stack is empty.

Narset, Enlightened Master — ruling 2

You can't play any land cards exiled with Narset.

Narset, Enlightened Master — ruling 3

Because you're already casting the card using an alternative cost (by casting it without paying its mana cost), you can't pay any other alternative costs for the card, including casting it face down using the morph ability. You can pay additional costs, such as kicker costs. If the card has any mandatory additional costs, you must pay those.

Narset, Enlightened Master — ruling 4

If the card has {X} in its mana cost, you must choose 0 as the value for X when casting it.

Narset, Enlightened Master — ruling 5

Any exiled cards you don't cast remain in exile.

Narset, Enlightened Master : règles, fonctionnement et interactions (MTG)

Narset, Enlightened Master is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

First strike, hexproof Whenever Narset attacks, exile the top four cards of your library. Until end of turn, you may cast noncreature spells from among those cards without paying their mana costs.

How it works : Narset, Enlightened Master

Original English text — Wizards of the Coast / Scryfall

#1September 20, 2014· Wizards of the Coast
The cards are exiled face up. Casting the noncreature cards exiled this way follows the normal rules for casting those cards. You must follow all applicable timing rules. For example, if one of the exiled cards is a sorcery card, you can cast it only during your main phase while the stack is empty.
No other card is named in this ruling.
#2September 20, 2014· Wizards of the Coast
You can't play any land cards exiled with Narset.
No other card is named in this ruling.
#3September 20, 2014· Wizards of the Coast
Because you're already casting the card using an alternative cost (by casting it without paying its mana cost), you can't pay any other alternative costs for the card, including casting it face down using the morph ability. You can pay additional costs, such as kicker costs. If the card has any mandatory additional costs, you must pay those.
No other card is named in this ruling.
#4September 20, 2014· Wizards of the Coast
If the card has in its mana cost, you must choose 0 as the value for X when casting it.
No other card is named in this ruling.
#5September 20, 2014· Wizards of the Coast
Any exiled cards you don't cast remain in exile.
No other card is named in this ruling.