The placement of the adverb "yesterday" in English sentences affects emphasis and clarity. When "yesterday" is at the beginning (like in "Yesterday, I went to the library and hired a much-needed book"), it emphasizes the timeframe right away. Placing it at the end (as in "I went to the library and hired a much-needed book yesterday") emphasizes the main action first and adds the timeframe afterward. Both ways tell us the action happened the day before today, but the placement changes which part of the sentence gets more focus. Overall, there isn't much of a difference, and both are used widely and it doesn't matter which one you use.