Learn Spanish (Latin America): El rey puso su tesoro de oro aquí.

Puso comes from the verb poner (to put). Poner is highly irregular in the preterite (completed past) tense. Instead of using a regular ending, its stem changes from pon- to pus-. For the 'he/she/it' form, it takes the irregular ending -o, making it puso (he put).

In English, 'put' can be present or past tense. Is puso only past tense?

Yes, puso is strictly past tense. To say 'The king puts' in the present tense, you use pone. Spanish is clearer than English here because the verb changes to tell you exactly when the action happened.

Why do we say tesoro de oro instead of 'oro tesoro' for 'gold treasure'?

In Spanish, you cannot stack two nouns together like we do in English with 'gold treasure' or 'coffee cup'. Instead, you link them with de (of). So, you literally say 'treasure of gold' (tesoro de oro).

Since su can mean 'his', 'her', 'your', or 'their', how do we know it means 'his' here?

Context! Because the sentence is about el rey (the king), we naturally assume the treasure belongs to him. If the context were ambiguous and you wanted to be extremely clear, you could say el tesoro de él (the treasure of him) instead of su tesoro.