Every time one "would" appears in a sentence, it means "conditional"?-- No. It has other uses, like the past of 'will' ('He told me that he would arrive at 7') and for past habit ('When I was a child, I would walk 10 km to school every day')
"What would your sample be used for."-what tense is the sentence above?-- This could be timeless present or it could be past
Here is the context. The sentence comes from a brochure (clinical trials) in which patients are explained what their samples will be used for. And I came across a sentence (title) that read:
"What your sample will be used for." And then it goes on to explain: "To help sceintists...."
And I "corrected" the sentence by saying "What would your sample
Well, the original is not correct as a question, but it is certainbly not a sentence-- it is just a noun phrase. If it is considered a question, then your revision changed the meaning-- you changed it from Conditional I (likely event) to Conditional II (unlikely event). It is not wrong; it is just different. The original, if a question, would read: 'What will your sample be used for