Breakdown of Этот инструмент используют строители.
этот
this [f: эта, n: это]
инструмент
the tool
строитель
builder
использовать
to use (+ accusative)
Questions & Answers about Этот инструмент используют строители.
Wait, the word order is 'This tool use builders.' Does the tool use the builders?
Not at all! Russian word order is very flexible, so the subject doesn't have to come first. Here, the object Этот инструмент comes first to emphasize it, as if saying 'This tool is the one builders use.' You have to rely on the grammar, not the word order, to know who is doing the action.
If Этот инструмент is the direct object, why didn't its ending change?
It actually is in the Accusative case, which is the case used for direct objects. However, because инструмент is a masculine inanimate noun (a non-living thing), its Accusative form looks exactly the same as its Nominative (dictionary) form. The same rule applies to the modifier Этот.
Since both nouns look like they could be the subject, how do we know for sure that строители is the subject?
The verb gives it away! The verb используют is in the 'they' form (plural). Since инструмент is singular (it) and строители is plural (they), the plural builders must be the ones doing the using.
If I wanted to add a relative clause to talk about 'the builders who use this tool,' which form of который would I use?
You would use которые (Nominative plural). When choosing the case for который, you only look at its job inside the relative clause. Since the builders are the subject doing the action in that clause, you would say: строители, которые используют этот инструмент.