This is 2nd person singular "tú," and thus requires the 2nd person singular conjugation of the verb "estar," being "estuviste."
thanks for the explanation. I was wondering why "tu" isn't first person singular?
That's not a common English phrase, no. "On the beach" describes a specific physical situation, i.e. "The whale is lying on the beach." or "After swimming for hours, I was finally standing on the beach." "At the beach" refers more to the general circumstances of the seaside.
I think this one could go either way. "Ayer" is yesterday - it's already ended. Yesterday also implies they weren't there the day before, or the day before that. They were just there yesterday, presumably for a definite number of hours. I think the preterite is best to use here, but a lot of sentences like this I think both would work.
What is the rule for the position of ayer and similar words? Do you always have to put it in the beginning of the sentence or can you follow the same word order as it is in English? Or is ayer the first words because it is emphasized?
Some of the hints will be correct, some will not. It's designed to give you enough of a hint that you could pick the right answer.
Could I write "Ayer tu estuviste a la playa"? Must I use "en" or could I use "a"?
Wouldn't you use estuve instead of estuviste? How can I tell the difference?