Learn Spanish (Latin America): ¿Quién te dijo que enterraron el oro en la tierra? - Who told you that they buried the gold in the ground?

decir (to say/tell) is highly irregular in the preterite (past) tense. Instead of a normal ending, the stem changes to dij-, and the he/she/it ending is -o. Since quién (who) is treated as a singular subject (just like 'he' or 'she'), we use dijo.

Why is te placed before dijo?

In Spanish, indirect object pronouns like te (to you) usually go immediately before the conjugated verb. So instead of putting 'to you' after the verb like in English, the Spanish structure is quién te dijo (who to you told).

Can I leave out que like we often drop 'that' in English?

No, you generally cannot skip it. In English, it is perfectly fine to say 'Who told you they buried the gold.' But in Spanish, the connecting word que is mandatory to link the two actions together.

Why use tierra for 'ground' instead of suelo?

tierra means dirt, earth, or land (as well as the planet Earth). Since burying gold involves digging into the dirt, tierra is the most natural word. suelo usually refers to the floor inside a building or the hard surface you walk on.