Learn Spanish (Latin America): Habían tallado un símbolo extraño en la piedra. - They had carved a strange symbol in the stone.

This sentence uses the past perfect tense to talk about an action that had already happened. We form it using the imperfect of haber (habían) plus the past participle (tallado). If you used han, it would mean they have carved instead of they had carved.

Is the verb tallar regular when forming the past participle?

Yes. To form the past participle tallado, you simply drop the -ar from tallar and add -ado. Notice that the past participle does not change its ending to match gender or number when it is used with a form of haber.

Why does the word símbolo have a written accent mark?

The word símbolo is stressed on the third-to-last syllable. In Spanish, words with this specific stress pattern always require a written accent mark to tell you exactly where to place the spoken emphasis.

Does extraño mean the same thing as the English word stranger?

They look similar, but extraño is an adjective that translates to strange or weird. If you want to talk about a stranger (a person you do not know), you would use the noun desconocido.