Yennett, Cryptic Sovereign — ruling 1

If the revealed card doesn't have an odd mana value or if that card does but you choose not to cast it, you draw a card. Keep in mind that revealing a card doesn't cause it to change zones. This means that the card you draw will be the card you revealed.

Yennett, Cryptic Sovereign — ruling 2

If you cast a card "without paying its mana cost," you can't choose to cast it for any alternative costs, such as evoke costs. You can, however, pay additional costs. If the card has any mandatory additional costs, such as that of Tormenting Voice, those must be paid to cast it.

Yennett, Cryptic Sovereign — ruling 3

If a spell has {X} in its mana cost, you must choose 0 as the value of X if you cast it without paying its mana cost.

Yennett, Cryptic Sovereign — ruling 4

If you wish to cast the revealed card without paying its mana cost, you must cast it while Yennett's triggered ability is resolving; you can't cast for free it later in the turn.

Yennett, Cryptic Sovereign — ruling 5

Because all attackers are chosen at once, a spell cast this way can't affect which creatures you can attack with, but it can affect which creatures will be able to block.

Yennett, Cryptic Sovereign : règles, fonctionnement et interactions (MTG)

Yennett, Cryptic Sovereign 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

Flying, vigilance, menace Whenever Yennett attacks, reveal the top card of your library. You may cast it without paying its mana cost if its mana value is odd. If you don't cast it, draw a card.

How it works : Yennett, Cryptic Sovereign

Original English text — Wizards of the Coast / Scryfall

#1July 13, 2018· Wizards of the Coast
If the revealed card doesn't have an odd mana value or if that card does but you choose not to cast it, you draw a card. Keep in mind that revealing a card doesn't cause it to change zones. This means that the card you draw will be the card you revealed.
No other card is named in this ruling.
#2July 13, 2018· Wizards of the Coast
If you cast a card "without paying its mana cost," you can't choose to cast it for any alternative costs, such as evoke costs. You can, however, pay additional costs. If the card has any mandatory additional costs, such as that of Tormenting Voice, those must be paid to cast it.
No other card is named in this ruling.
#3July 13, 2018· Wizards of the Coast
If a spell has in its mana cost, you must choose 0 as the value of X if you cast it without paying its mana cost.
No other card is named in this ruling.
#4July 13, 2018· Wizards of the Coast
If you wish to cast the revealed card without paying its mana cost, you must cast it while Yennett's triggered ability is resolving; you can't cast for free it later in the turn.
No other card is named in this ruling.
#5July 13, 2018· Wizards of the Coast
Because all attackers are chosen at once, a spell cast this way can't affect which creatures you can attack with, but it can affect which creatures will be able to block.
No other card is named in this ruling.