The mana value of a split card is determined by the combined mana cost of its two halves. If cascade allows you to cast a split card, you may cast either half but not both halves.

Cascade triggers when you cast the spell, meaning that it resolves before that spell. If you end up casting the exiled card, it will go on the stack above the spell with cascade.

When the cascade ability resolves, you must exile cards. The only optional part of the ability is whether or not you cast the last card exiled.

If a spell with cascade is countered, the cascade ability will still resolve normally.

You exile the cards face up. All players will be able to see them.

If you cast a card "without paying its mana cost," you can't choose to cast it for any alternative costs. You can, however, pay additional costs. If the card has any mandatory additional costs, you must pay those to cast the card.

If the card has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

Due to a 2021 rules change to cascade, not only do you stop exiling cards if you exile a nonland card with lesser mana value than the spell with cascade, but the resulting spell you cast must also have lesser mana value. Previously, in cases where a card's mana value differed from the resulting spell, such as with some modal double-faced cards or cards with an Adventure, you could cast a spell with a higher mana value than the exiled card.

A spell's mana value is determined only by its mana cost. Ignore any alternative costs, additional costs, cost increases, or cost reductions.

Sakashima's Protege copies exactly what was printed on the original permanent (unless that permanent is copying something else or is a token; see below). It doesn't copy whether that permanent is tapped or untapped, whether it has any counters on it or any Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, or so on.

If Sakashima's Protege enters the battlefield as a copy of a planeswalker, it will enter with a number of loyalty counters on it equal to the loyalty printed in the lower right corner of the planeswalker. It won't copy the number of counters currently on that planeswalker.

If the chosen permanent is copying something else (for example, if the chosen permanent is another Sakashima's Protege), then Sakashima's Protege enters the battlefield as whatever the chosen permanent copied.

If the chosen permanent is a token, Sakashima's Protege copies the original characteristics of that token as stated by the effect that put that token onto the battlefield. Sakashima's Protege doesn't become a token in this case.

Any enters-the-battlefield abilities of the copied permanent will trigger when Sakashima's Protege enters the battlefield. Any "as [this permanent] enters the battlefield" or "[this permanent] enters the battlefield with" abilities of the chosen permanent will also work.

If Sakashima's Protege somehow enters the battlefield at the same time as another creature, it can't become a copy of that creature. You may choose only a creature that's already on the battlefield and entered the battlefield this turn.

If you cast a permanent spell using the cascade ability of Sakashima's Protege, that permanent will be on the battlefield in time to be copied by Sakashima's Protege.