Learn Arabic (Modern Standard): ترك الجد ثروة كبيرة.

In Arabic, the most common sentence structure is Verb-Subject-Object. So instead of starting with 'The grandfather left,' it is much more natural in Modern Standard Arabic to put the action first and say 'Left the grandfather.'

Why is the adjective كبيرة feminine when the grandfather is a man?

Adjectives always describe the noun right before them. Here, كبيرة describes the wealth (ثروة), not the grandfather. Because ثروة is a feminine noun ending in a 'taa marbuuta' letter, its adjective must also take a feminine ending to match it.

Does the verb ترك mean 'to leave' a room, or 'to leave' something behind?

The verb ترك means to leave something behind, abandon it, or leave an inheritance. If you want to say the grandfather left a physical place or departed, you would usually use a different verb, such as غادر.

Why don't the words ثروة كبيرة have the prefix ال attached to them?

Because the meaning is 'a great wealth,' not 'the great wealth.' In Arabic, when a noun is indefinite, it does not take ال. Furthermore, adjectives must always match their nouns in definiteness, so the adjective drops the prefix too.