Сборка мебели кажется простой, но без хорошего инструмента она занимает несколько часов.

Assembling furniture seems simple, but without a good tool it takes several hours.

Breakdown of Сборка мебели кажется простой, но без хорошего инструмента она занимает несколько часов.

• хороший: good
• но: but
• она: it
• без: without

Questions & Answers about Сборка мебели кажется простой, но без хорошего инструмента она занимает несколько часов.

Why is it кажется простой and not кажется простая?

After казаться (to seem), Russian often uses the instrumental case for the “result/predicate” description:

• Сборка мебели кажется простой. = The furniture assembly seems simple.
  Here простой is instrumental feminine singular (agreeing with сборка, which is feminine).
  You can sometimes see nominative too (especially in some styles), but instrumental is very common and often preferred with казаться.

What does Сборка мебели literally mean, and why is мебели in that form?

Сборка мебели literally means assembly of furniture.
мебели is genitive because Russian commonly uses the genitive to show “of” / what something relates to:

• сборка (чего?) мебели = assembly (of what?) furniture.
  Also, мебель is usually a mass noun (“furniture” as a category), so genitive singular мебели is normal here.

Why does the sentence repeat with она? Could it be omitted?