Learn Spanish (Latin America): Por más que tuvieron cuidado, dejaron el esmalte nuevo agrietado. - No matter how careful they were, they left the new enamel cracked.

This is a resultative periphrasis, using the verb dejar (to leave) followed by a past participle (agrietado). It emphasizes the final consequence or state of an action. Instead of just saying they cracked it (lo agrietaron), dejaron el esmalte... agrietado highlights the ruined state they left the enamel in. Remember that the past participle acts as an adjective in this structure, so it must agree in gender and number with the direct object (el esmalte).

Why is tuvieron in the indicative instead of the subjunctive? Doesn't por más que trigger the subjunctive?

Por más que (no matter how much / even though) only triggers the subjunctive when the situation is hypothetical, unknown, or in the future. Because this sentence states a known, completed fact in the past (they actually were careful), we use the indicative tuvieron. If you were talking about a hypothetical future situation (no matter how careful they might be), you would use the subjunctive: por más que tengan cuidado.

Why do we use tuvieron cuidado instead of something like fueron cuidadosos?

While fueron cuidadosos is grammatically valid, Spanish heavily prefers using the verb tener (to have) followed by a noun to express states of being where English uses the verb to be plus an adjective. Therefore, to be careful translates directly to the very common phrase tener cuidado (literally, to have care).