siktak wie jageun mulgeondeuri manhaseo jeongrihaeya haeyo.

Use -에 for stating where something is or exists (“at/on”). Use -에서 when you describe where an action takes place or when you talk about the starting point of movement: • 식탁 위에 작은 물건이 있어요. (Static existence: there are things on the table.)
• 식탁 위에서 밥을 먹어요. (Action location: I eat on the table.)
In our sentence, we’re describing where the objects are, so 에 is correct.

The suffix -들 marks plurality (“items” rather than “item”). It’s optional in Korean because context often makes number clear.
• With 들: 작은 물건들이 많아서… (emphasizes “many small items”)
• Without 들: 작은 물건이 많아서… (still understood as “many small things”)
Adding 들 simply clarifies that you’re talking about multiple objects.

-아서/어서 is a connective ending that links two clauses by showing cause or reason. In 많아서, it means “because (there are) many.” So the first clause explains why the second clause occurs:
“Because there are many small items on the table, I need to tidy up.”

-아/어야 하다 expresses necessity or obligation (“have to/need to”).
• 정리하다 = “to tidy up.”
• 정리해야 하다 = “to have to tidy up.”
Adding -요 for politeness gives 정리해야 해요, meaning “I have to tidy up.”

In everyday Korean, 해야 돼요 and 해야 해요 both mean “have to do.” They’re interchangeable in most contexts.
• 해야 돼요 can feel like an external requirement or rule.
• 해야 해요 might feel more like your own decision or plan.
In practice, native speakers use them almost identically.