Ryan D. answered 12/07/20
English & Spanish Fluent Speaker
Hi, from my experience the word "ayer" can be used before or after. In other words,
la palabra "ayer", can be used in many forms. While the comment above is also correct, speaking to native speakers you will notice they never include "Tu" in that phrase. For example
tu los compraste ayer?, as stated above but the common way is -> los compraste ayer? or ayer los compraste?
Both are correct and when speaking if you say "ayer tu los compraste?", its a normal response BUT in a fluent form, the more common way to say it is "los compraste ayer?" with respect to how people talk day to day. The word "Compraste" because of the ending, (ste) we know the word is 2 things. Past tense AND directed to you or whoever you are talking to.