Learn Spanish (Latin America): Ustedes pusieron unas piedras grandes en la tumba.

The verb poner (to put) is highly irregular in the preterite (simple past) tense. Instead of keeping its normal stem, it changes to pus-. Since Ustedes is the subject, we add the ending -ieron to make pusieron.

In English, "put" is the same in the present and past tense. Is that true in Spanish?

No, Spanish uses different forms! If you want to say "You (plural) put" right now in the present tense, you would say Ustedes ponen. In this sentence, pusieron clearly tells us that the action already happened in the past.

Why does grandes come after piedras, and why doesn't it end in "-as" to match the feminine word?

In Spanish, adjectives usually come after the noun they describe, so we say piedras grandes (stones large). Also, adjectives that end in -e in their singular form (like grande) don't have separate masculine and feminine forms. To make them plural, you just add -s whether the noun is masculine or feminine.

The English translation says "on the tomb", but doesn't en mean "in"?