Spider-Sense

Web-slinging represents an alternative cost. If another effect allows you to play a card for a different alternative cost, including "without paying its mana cost," you can't also choose to pay its web-slinging cost. If any additional costs from other sources are applied, you must pay those.

Triggered abilities use the words "when," "whenever," or "at." They're often written as "[Trigger condition], [effect]." Some keyword abilities (such as prowess) are triggered abilities and will have "when," "whenever," or "at the beginning of" in their reminder text.

If any permanent has an ability that triggers when the permanent you return leaves the battlefield, that ability will be put on the stack after you finish casting the spell, and it will resolve before that spell.