Learn Finnish: Pihditkö aiheuttivat särön kristalliin? - Was it the tongs that caused the crack in the crystal?

By adding the question clitic -kö to the noun pihdit and moving it to the front of the sentence, you are using contrastive focus. You aren't just asking if a crack was caused; you are specifically verifying what caused it: 'Was it the tongs (and not something else)?'

Since we are likely talking about a single pair of tongs, why is the verb aiheuttivat in the plural?

The word pihdit (pliers/tongs) is a word that only exists in the plural form, much like 'scissors' in English. Because the subject is grammatically plural, the verb must match it and take the third-person plural form aiheuttivat.

Why is the object särön in the genitive-accusative form instead of the partitive säröä?

The verb aiheuttaa (to cause) here describes a completed action resulting in a specific outcome: a whole crack was formed. Because the result is complete and the sentence is affirmative, Finnish requires a total object, which takes the genitive-accusative ending -n.

English says the crack is 'in' the crystal. Why does Finnish use the illative case (kristalliin) rather than the inessive (kristallissa)?

In Finnish, when you create a mark, hole, dent, or crack on or in something, the damage is conceptually thought of as going into the object. Therefore, verbs describing the creation of structural damage often require the target to take the illative case to show the direction of the change.