Breakdown of Внутри сейфа лежало завещание.
лежать
to lie, be lying down
сейф
safe
внутри
inside (location)
завещание
the will
Questions & Answers about Внутри сейфа лежало завещание.
Why is the subject (завещание) placed at the very end of the sentence?
In Russian, the end of a sentence is usually reserved for the "new" or most important information (the Rheme). Because Russian doesn't have articles like "a" or "the", word order often does that job. Putting завещание at the end implies it is "a will" (new information we just discovered). If the sentence were Завещание лежало внутри сейфа, it would mean "The will was inside the safe" (we already knew about the will, but we are just now learning its location).
Does placing the subject at the end change how the verb agrees with it?
Not at all. The verb лежало is in the neuter past tense to agree with the subject завещание (a neuter noun). The grammatical agreement remains exactly the same whether the subject comes before or after the verb.
Why do we use лежало (lay) here instead of just было (was)?
While you could technically use было, Russian strongly prefers verbs of physical position (лежать, стоять, висеть) to describe where objects are located. Since a document like a will is a flat object, it "lies" (лежит) in the safe.
Why is сейф in the genitive case (сейфа)?
The preposition внутри (inside) always requires the following noun to be in the genitive case.