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You must follow the normal timing permissions and restrictions for the top card of your library and pay its costs. If the card has alternative or additional costs, you may pay those. If the card has any mandatory additional costs, you must pay those.
The top card of your library isn't in your hand, so you can't suspend it, cycle it, or activate any of its abilities.
If the top card of your library changes while you're casting a spell, playing a land, or activating an ability (most likely because that top card is the card you're playing), the new top card won't be revealed until you finish doing so.
If the top card of your library is a land, you may play that land only if you have any available land plays.
